---
abstract: |
    We present an analysis of ground-based optical photometry and spectroscopy, and [*Rossi X-ray Timing Explorer*]{} X-ray observations of the old novae DI Lacertae and V841 Ophiuchi. Our optical photometry data (obtained with the automated photometry telescope RoboScope) comprise an almost decade-long light curve for each star, while the contemporaneous spectroscopy and X-ray observations repeatedly sampled each nova during separate intervals of $\approx45$–55 d in length. The long-term optical light curves of both novae reveal quasiperiodic variability on typical time scales of $\sim30$–50 d with amplitudes of $\Delta V\sim0.4$–0.8 mag. V841 Oph also displays a long-term, sinusoidal modulation of its optical light on a time scale of 3.5–5 yr. The optical spectra of these novae display quite different characteristics from each other, with DI Lac showing narrow Balmer emission cores situated in broad absorption troughs while V841 Oph exhibits strong single-peaked Balmer, and emission lines. We find little change between spectra obtained during different optical brightness states. The X-ray count rates for both novae were very low ($\lesssim1.5$ ct s$^{-1}$) and there was no reliable correlation between X-ray and optical brightness. The combined X-ray spectrum of DI Lac is best fit by a bremsstrahlung emission model (with $kT\sim4$ keV and $N_{\rm H} < 1.8\times10^{22}$ cm$^{-3}$); the X-ray spectrum of V841 Oph is too weak to allow model fitting. We discuss the possible origin of variability in these old novae in terms of magnetic activity on the secondary star, dwarf nova type disk instabilities, and the “hibernation” scenario for cataclysmic variable stars.

    [Accepted by PASP on 28 August 2000 for the December 2000 issue.]{}
author:
- |
    D. W. Hoard, Paula Szkody, R. K. Honeycutt, Jeff Robertson, Vandana Desai,\
    and T. Hillwig
title: 'Long-term optical and X-ray observations of the old novae DI Lacertae and V841 Ophiuchi'
---

Introduction {#s-intro}
============

Cataclysmic variables (CVs) are semi-detached interacting binary stars composed of a white dwarf (WD) primary star and a low mass ($\lesssim0.5$M$_{\odot}$) main sequence secondary star, with typical orbital periods of $\lesssim1$ d. The Roche-lobe-filling secondary star loses mass through the inner Lagrangian point into an accretion disk formed around the WD (for non-magnetic CVs). Classical novae are a subclass of CV in which a thermonuclear runaway is triggered in a reservoir of matter that has been gradually accreted onto the WD. The resultant outburst produces a peak brightness increase of $\approx$ 6–15 mag, and releases $10^{44}-10^{46}$ erg [see review in @Warn95 ch. 5].

The outbursts of novae are often well-observed, and the long term behavior of many dwarf novae and novalike CVs is monitored by the American Association of Variable Star Observers. However, there have been few programs for monitoring the variability of novae at quiescence. One such program, using an automated telescope called RoboScope (see §\[s-robo\]), has monitored 22 old novae and 42 novalikes, most of them for over 9 years now. RoboScope has found several kinds of unusual photometric behavior. About one-third of the old novae have shown quasi-periodic variability for a year or two, interspersed with stable light curves. These variations do not appear stochastic as they repeat at similar periods when they reappear. Preliminary analysis of RoboScope light curves spanning several years for the old nova DI Lacertae suggested brightness oscillations on a time scale of $\approx35$ d [@Hone95], while the old nova V841 Ophiuchi also displayed prominent variability [@Hone94]. In order to determine if these oscillations could be related to a disk instability mechanism (as operates in dwarf novae) or to mass transfer or magnetic effects on the secondary star, we attempted a detailed study of DI Lac and V841 Oph, which are among the most active and brightest of the systems showing the oscillations.

DI Lac (= Nova Lac 1910) was a moderately fast nova that reached a maximum brightness of $m_{V}({\rm max}) = 4.3$ mag during its outburst. V841 Oph (= Nova Oph 1848) was a slow nova that reached a similar brightness, $m_{V}({\rm max}) = 4.2$ mag [@Kuka71]. The reddening for both objects has been measured from [*International Ultraviolet Explorer*]{} ($IUE$) ultraviolet and ground-based optical spectra. The range for DI Lac is $E(B-V)=0.15$–0.41 [@Bruc84; @Cass90], while for V841 Oph it is $E(B-V)=0.30$–0.58 [@Bruc84; @Cass90; @Weig94; @Verb97]. Nova shells were not detected in H$\alpha$ for either CV [@Cohe85]. In X-rays, DI Lac was undetected in the $ROSAT$ PSPC All Sky Survey, with a $2\sigma$ upper limit of 0.013 cts s$^{-1}$ in the 11–201 channel (0.1–2.0 keV) energy range during 299 s total exposure time, while V841 Oph was a marginal detection with $0.017\pm0.007$ cts s$^{-1}$ in the “hard” 0.9–2.0 keV [*ROSAT*]{} energy range ($0\pm0.025$ cts s$^{-1}$ in the full [*ROSAT*]{} energy range) during 402 s total exposure time [@Verb97].

We report here on the results of ground-based optical photometry and spectroscopy, and [*Rossi X-ray Timing Explorer*]{} ($RXTE$) X-ray observations of DI Lac and V841 Oph. The optical photometry comprises an almost decade-long light curve for each star, while the contemporaneous spectroscopy and X-ray observation repeatedly sampled each nova during separate intervals of $\approx45$–55 d in length.

Observations and Analysis
=========================

Our long-term optical photometry of DI Lac and V841 Oph was accomplished from 1990–1998, while the spectroscopic amd X-ray observations took place in 1997. These observations are summarized in Tables \[t-DI\_log\] and \[t-V8\_log\], and are discussed in detail below.

Optical Photometry {#s-robo}
------------------

Our optical photometry data were acquired by RoboScope, a 41-cm telescope in Indiana equipped for automated differential CCD stellar photometry [@HT92]. All observatory operations (including data reductions) are accomplished as fully unattended and unsupervised tasks, which makes practical the acquisition of long homogeneous data streams. Typically, RoboScope obtains one or two 4-min exposures per clear night for each of $\sim140$ program stars. The data are reduced using the method of incomplete ensemble photometry [@Hone92]. For DI Lac, 85 ensemble stars were used in 987 exposures over 9 observing seasons from 1990 November to 1998 November. For V841 Oph, 24 ensemble stars were used in 611 exposures over 8 observing seasons from 1991 May to 1998 September. The zero-points of the differential light curves were established using secondary standard stars from @Hend95 [@Hend97]. Six such secondary standards were used for V841 Oph, while 13 were employed for DI Lac. Typical $1\sigma$ uncertainties in the calibrated magnitudes are on the order of 0.01–0.05 mag. Over the entire multiyear range of the RoboScope light curves, the mean magnitudes were $V\approx14.55$ for DI Lac and $V\approx13.6$ for V841 Oph, with full ranges of variability of $\Delta V\approx0.9$ mag and $\Delta V\approx1$ mag, respectively. The RoboScope light curves of DI Lac and V841 Oph are shown in Figures \[f-DI\_lc\] and \[f-V8\_lc\].

With only limited excursions to anomalously fainter magnitudes, the typical minimum magnitude of $\approx14.7$ for DI Lac has a sharp boundary at any given epoch. However, the minimum brightness displays a slight, apparently linear, trend towards increasing brightness over the 8 years of RoboScope observation. If we ignore for the moment the small number of data points that fall below the well-defined low brightness limit seen in the top panel of Figure \[f-DI\_lc\], then the typical minimum brightness of DI Lac changes from $V\approx14.75$ mag at the beginning of the RoboScope coverage, to $V\approx14.67$ mag at the end. This corresponds to a mean increase of the minimum brightness by $\Delta V\approx -0.01$ mag yr$^{-1}$. The maximum brightness of DI Lac is much less uniform, and it is difficult to determine if the maximum brightness behaves in the same manner as the minimum brightness. A linear fit to the complete DI Lac light curve (with each data point weighted by the inverse square of its $1\sigma$ uncertainty) gives the rate of change of the mean magnitude as $< +0.001$ mag yr$^{-1}$; that is, the mean brightness of DI Lac is essentially constant. However, if we exclude the 2 excursions to fainter magnitudes at HJD 2450600 and HJD 2451000 (as well as the excursion to brighter magnitudes at HJD 2450300), then the slope of the linear fit changes to $\Delta V = -0.004$ mag yr$^{-1}$; that is, a trend towards increasing mean brightness.

The faint magnitude excursions in the light curve of DI Lac are very similar to the “dips” seen by @Hone98 in long-term RoboScope light curves of five old novae and novalike CVs. In the latter systems, the dips were found to often be paired with a preceding or following outburst, although the dips also sometimes occurred as isolated events. @Hone98 do not find a clear mechanism (e.g. disk instabilities, truncated disks, mass transfer modulation) that is responsible for the dips, but conclude that the overall photometric behavior (dips and outbursts) of the novae and novalikes that they studied is likely to be governed by either a combination of disk and mass transfer events, or by mass transfer events alone.

In contrast to the linear trend in the minimum brightness of DI Lac, the minimum brightness of V841 Oph appears to follow an almost sinusoidal trend that completes approximately 1.5–2 cycles during the 7.5 year RoboScope coverage. As seen in the top panel of Figure \[f-V8\_lc\], the minimum brightness of V841 Oph varies from $V\approx14.5$ mag to $V\approx13.8$ mag. The maximum brightness of V841 Oph, which ranges from $V\approx13.1$ mag to $V\approx13.4$ mag, displays the same behavior; that is, when the minimum brightness at a given epoch is at its faint (bright) level, the maximum brightness is also at its faint (bright) level. We note that the magnitude range spanned by the minimum brightness in V841 Oph is about twice as large as that spanned by the maximum brightness. Although a linear fit to this overall trend is perhaps not the optimum choice, the corresponding rate of mean magnitude change is $\Delta V = +0.010$ mag yr$^{-1}$. It is possible (but not required by the data!) that the apparently linear trend in the light curve of DI Lac may also be a sinusoid, but with a much longer cycle length than in V841 Oph.

We searched for periodicities in the range 10–100 days in the RoboScope light curve data in two ways: by applying the phase dispersion minimization (PDM) algorithm [@Stel78], as well as performing an independent power spectrum analysis using the [clean]{} algorithm [@Robe87]. We performed the period search on both the entire data set for each CV, as well as on the data subsets from individual observing seasons (indicated in Figures \[f-DI\_lc\] and \[f-V8\_lc\]). The most prominent period detected in each data set is listed in Table \[t-periods\] (in several cases, most notably for the combined data sets for each nova, two significant periods are listed). In general, the two period search methods gave equivalent results to within better than 1–4 days. All of the detected time scales appear to correspond to quasiperiodic behavior rather than truly periodic variability such as that resulting from, for example, an eclipsing orbit. The time scales have significant “flexibility” (on the order of 1–5 days for periods $\lesssim50$ d, 5–20 days for periods $\gtrsim50$ d) over the multi-year length of the light curves (as shown by substantially broadened dips and peaks in the PDM and [clean]{} analyses, respectively). Phase-binned light curves of both CVs folded on the shorter period found for each combined data set are shown in Figure \[f-folded\_lc\]. We note that the true amplitude of variation in a given cycle or observing season will be larger than the 0.1–0.2 mag suggested by Figure \[f-folded\_lc\], in which data from many cycles with slightly different amplitudes and periods have been averaged together. Inspection of Figures \[f-DI\_lc\] and \[f-V8\_lc\] shows amplitudes of 0.4–0.8 mag for the full range of variability within each observing season. The long-term sinusoidal trend in the light curve of V841 Oph is also well-fit by a periodicity in the range 1800–1900 d or, with slightly less agreement, 1250–1300 d. (We note that these two period ranges are approximately related by the ratio 3:2, so we are likely seeing an aliasing effect due to the fact that only 1.5–2 cycles of these long periods are contained in the light curves.)

Optical Spectroscopy {#s-optspec}
--------------------

We obtained optical spectra of both CVs using the Double Imaging Spectrograph on the Apache Point Observatory (APO) 3.5-m telescope [@Gill95][^1] during University of Washington share time. Spectra of DI Lac were obtained in July and August of 1997; spectra of V841 Oph were obtained in May of 1997. The APO spectra have a resolution of $\approx2$Å and cover simultaneous wavelength ranges of $\approx4200$–5000Å and $\approx5800$–6800Å. The raw spectrum images were reduced in the standard fashion using IRAF. The instrumental response was removed via spectra of standard stars [@Mass88] obtained on the same nights; however, slit losses due to guiding errors rendered the absolute flux calibration unreliable. Consequently, we have normalized the spectra to a constant continuum level of 1.0.

A representative spectrum of DI Lac is shown in Figure \[f-DI\_spec\].

Note the prominent Balmer absorption troughs containing narrow, central emission cores. Our other two APO spectra of DI Lac (not shown) are essentially identical to this one, with the exception that the Balmer emission cores in the August spectrum are somewhat stronger (relative to ) than in the July spectra. The $\lambda4471$ feature (see inset) is present with the same profile shape in all three of our spectra. It is suggestive of the absorption trough with central emission as seen in the Balmer lines. The feature labeled with “?” is likely an instrumental or reduction artifact since its wavelength does not match any identifiable line and it occurs in [*all*]{} of our APO spectra (those of V841 Oph included).

Additional optical spectra of both novae were obtained using the Hydra Multi-fiber Positioner + Bench Spectrograph [@Bard92; @Bard94] on the WIYN 3.5-m telescope[^2]. Spectra of DI Lac were obtained in 1997 September, and spectra of V841 Oph were obtained in 1997 May. The WIYN spectra have a resolution of $\approx1$Å and cover (usable) wavelength ranges of $\approx6200$–6800Å for DI Lac and $\approx4400$–5000Å  for V841 Oph. For the WIYN spectra of DI Lac, no standard star observations were available, so it was not possible to correct for the instrumental response or perform a flux calibration. We do not show these spectra here, but note that they are qualitatively similar to the red APO spectrum shown in Figure \[f-DI\_spec\] – no differences corresponding to the different brightness levels in the RoboScope light curve are apparent. The V841 Oph WIYN data were reduced and calibrated as with the APO spectra. Again, slit losses and, additionally, the presence of cirrus made the absolute flux calibration unreliable, so we have normalized the spectra to a constant continuum level of 1.0. All of the spectra of V841 Oph are shown in Figure \[f-V8\_spec\] (the three WIYN spectra have been averaged together to improve the S/N).

There is no indication of the absorption troughs seen in the spectra of DI Lac. Additionally, the / + emission complex appears to be stronger, with broader lines, in V841 Oph than in DI Lac.

Flux-calibrated optical spectra of both DI Lac and V841 Oph are shown in @Will83, with resolutions of 5.1Å and 6.7Å, respectively, and in @Ring96, with resolution of 15–20Å for both. @Will83 obtained his spectra in 1981, while @Ring96 obtained theirs in 1990. In both papers, the spectra of both novae show steep blue continua; the observed continuum of DI Lac is bluer than that of V841 Oph (but different reddening values could eliminate or reverse this characteristic). @Ring96 fit a power law of the form $F_{\lambda} \propto \lambda^{-\alpha}$ to the continuum regions 5600–6500Å and 6650–8000Å in their spectra. These fits yielded indices of $\alpha=1.81(2)$ – dereddened to $\alpha_{0}=3.10(3)$ using $E(B-V)=0.41$ – for DI Lac and $\alpha=1.45(1)$ – dereddened to $\alpha_{0}=2.79(1)$ using $E(B-V)=0.58$ – for V841 Oph. @Cass89 performed this power law fit to the wavelength region 1200–3200Å in $IUE$ UV spectra of DI Lac and V841 Oph. Using reddenings at the low ends of the ranges given in §\[s-intro\], $E(B-V)=0.15$ for DI Lac and $E(B-V)=0.30$ for V841 Oph, they calculated dereddened slopes of $\alpha_{0}=1.5$ and $\alpha_{0}=2.0$, respectively. Comparison with the slope expected for the continuum flux distribution of a steady-state accretion disk composed of parcels radiating as blackbodies, $\alpha=2.3$ [@Prin81], suggests that the true reddening values for DI Lac and V841 Oph are near the middle of the ranges given in §\[s-intro\].

In 1936, both of these novae were observed spectroscopically by @Huma38, who noted their strong blue continua, but also noted the absence of [*any*]{} lines, emission or absorption. Since that time, the available published spectra of DI Lac and V841 Oph in @Gree60, @Will83, and @Ring96 are consistent with our current spectra [see spectroscopic histories summarized in @Ring96], with the exception that @Will83 did not report the detection of any $\lambda4686$ emission in either star.

X-ray Observations
------------------

DI Lac and V841 Oph were each observed ten times during July–September and April–June of 1997, respectively, with the Proportional Counter Array [PCA; @Jaho96] on the $RXTE$ satellite [e.g. @Brad93][^3]. The PCA consists of five xenon-methane proportional counters effective over the range 2–60 keV (with 18% energy resolution at 6 keV). Each $RXTE$ visit constituted a Good-Time-Interval on-source lasting $\approx1000$–2500 s. We extracted the net average X-ray count rate from each visit and the X-ray spectrum from the ten combined visits to each star using the FTOOLS/LHEASOFT (v5.0) software package. We utilized the faint source background models (pca\_bkgd\_faintl7\_e3v19990824.mdl and pca\_bkgd\_faint240\_e3v19990909.mdl) and an up-to-date South Atlantic Anomaly history file (pca\_saa\_history\_20000223). Extraction of the count rates and spectra was carried out as described in “The RXTE Cookbook”[^4].

The X-ray light curves constructed from the mean count rates at each visit for DI Lac and V841 Oph are shown in Figures \[f-DI\_Xray\] and \[f-V8\_Xray\], respectively, along with the corresponding sections of their RoboScope $V$ band light curves.

The light curve counts were summed in the energy range 0–15 keV. Inspection of the X-ray spectra (see below) revealed an essentially zero count rate dominated by noise at energies higher than 15 keV. The mean X-ray count rates averaged over all ten visits to each target are $1.30\pm0.32$ ct s$^{-1}$ for DI Lac and $0.56\pm0.73$ ct s$^{-1}$ for V841 Oph. Considering these very low count rates, it is not clear whether or not the variability seen in the X-ray light curves is real.

Additional analysis and model-fitting (DI Lac only) of the X-ray spectra of these novae was performed using the routine XSPEC (v11). Although we extracted the full range of available energy channels in the X-ray spectra, only the range 0–15 keV was used for fitting the models to the DI Lac data. The X-ray spectra for DI Lac and V841 Oph are shown in Figures \[f-DI\_modspec\] and \[f-V8\_Xspec\].

The X-ray spectrum of V841 Oph is shown on the same scale as that of DI Lac; the former is clearly much weaker than the latter. We attempted to extract a better spectrum of V841 Oph by using only the data from the four visits with the largest count rates (visits 3, 5, 9, and 11 – see Table \[t-V8\_log\]). Unfortunately, this spectrum was virtually identical to the total combined spectrum (i.e. flat and featureless), with the exception of a small upward offset of the mean count rate, from about 0.02 cts s$^{-1}$ keV$^{-1}$ in the total combined spectrum to about 0.04 cts s$^{-1}$ keV$^{-1}$ in the “high” count rate spectrum. Consequently, no attempt was made to fit a model to the spectrum of V841 Oph.

For DI Lac, we fit three simple models to the X-ray spectrum: (1) blackbody, (2) Raymond-Smith thermal plasma, and (3) bremsstrahlung. Each model was modified by a multiplicative component representing photoelectric (H column) absorption. More complex models were not warranted owing to the small number of counts per energy channel in the data. In all cases, the data were weighted using the XSPEC-recommended scheme appropriate for small count numbers, $W_{\rm i}=1+(N_{\rm i}+0.75)^{0.5}$ [@Gehr86]. The results of these model fits are summarized in the first three rows of Table \[t-DI\_model\]. All of the models produce nearly indistinguishable fits to the observed spectrum. The bremsstrahlung model is formally the best-fitting (with lowest reduced $\chi^{2}$ and highest null hypothesis probability) and is plotted over the observed spectrum in Figure \[f-DI\_modspec\].

All of the models were only able to constrain the hydrogen column density to an upper limit; the values of $N_{\rm H}$ quoted in the first three rows of Table \[t-DI\_model\] are the upper ends of the 1-parameter 90% confidence intervals. We used the relation $$N_{\rm H} = E(B-V) \times 5.8\times10^{21} {\rm cm}^{-2} {\rm mag}^{-1}$$ [@Bohl78] to estimate values of $N_{\rm H}$ spanning the reddening range given in §\[s-intro\]. We then re-fit the models to the X-ray spectrum of DI Lac with only two free parameters: temperature and normalization. The results are listed in the bottom six rows of Table \[t-DI\_model\]. While slightly different from those obtained from the original model fits, they are completely consistent with them. The bremsstrahlung model yields a slightly lower nominal temperature of $kT\approx4.1$–4.2 keV (vs. $kT=4.5$ keV when $N_{\rm H}$ is a free parameter) and is still the best-fitting model for both values of $N_{\rm H}$.

We note that the uncertainties quoted in Table \[t-DI\_model\] for $kT$ and $C_{\rm norm}$ are 1-parameter 90% confidence intervals; the interdependent parameter uncertainties will be slightly different. For example, for the bremsstrahlung model (with free $N_{\rm H}$), the XSPEC routine steppar[^5] gives a 2-parameter 90% confidence interval of $$kT = 4.5^{+1.9}_{-1.5} {\rm ~keV}$$ for $kT$ with respect to $N_{\rm H}$, and $$kT = 4.5^{+1.8}_{-1.7} {\rm ~keV}$$ for $kT$ with respect to $C_{\rm norm}$.

The X-ray count rates were too low to reliably fit spectrum models to any of the ten individual $RXTE$ visits to DI Lac. We constructed “low” and “high” state X-ray spectra by combining the data from visits with the lowest mean count rates (1, 2, 3, 4, 7) and from visits with the highest mean count rates (5, 6, 9) to see if there were any differences between the low and high X-ray brightness states[^6]. Other than an $\approx1$ cts s$^{-1}$ keV$^{-1}$ increase at the peak energy of the high state spectrum, the two spectra are essentially indistinguishable from each other (and from the combined spectrum from all ten visits). We conclude from this that either the spectral energy distribution of the X-ray emitting source in DI Lac does not change as the overall brightness of the X-ray source changes, or that the variability seen in the X-ray light curve of DI Lac is not real (i.e. it is a side-effect of the very low count rates).

Discussion
==========

Optical Photometric Variability {#s-optvar}
-------------------------------

Inspection of the RoboScope light curves of DI Lac and V841 Oph reveals variability on three distinct time scales. First, in the range 10–100 d, quasiperiods with typical lengths of $\sim30$–50 d are present in each of these CVs. Yet, the presence (or lack) of periodic and/or quasiperiodic variability on time scales up to a few hundred days in V841 Oph has been contested in the literature. (To the best of our knowledge, no extensive database of photometric observations of DI Lac has been published prior to this work.) Using 420 archival visual observations of V841 Oph spanning 28 yr, @Dell87 quote an average period of 51.5 d with individual cycles ranging from 45 d to 57 d. @Shar89 observed V841 Oph once per night for 31 nights during a 45 night interval in 1986 using a CCD camera + $B$ filter, and found a period of $\sim40$ d. All of these values are comparable to the $\sim35$–50 d we found for our complete RoboScope light curve of V841 Oph, as well as to the range of periods found for individual observing seasons in the RoboScope data (see Table \[t-periods\]). On the other hand, @Duer92 notes the presence of considerable variability in archival light curves of V841 Oph [including some of the same data used by @Dell87], but finds no evidence for periodicities up to 200 d.

It is quite interesting that the variability time scales in these two novae are similar to those that characterize dwarf nova outbursts [@Warn95 ch. 3]. Old novae are presumed to have mass transfer rates, $\dot{M}$, above the threshold level ($\dot{M}_{\rm crit}$) at which the accretion disk thermal instability mechanism operates in dwarf novae to produce their outbursts [@Osak96]. However, the hibernation theory of cyclic evolution between CV variability types [@Sh89 and references therein], predicts that when $\dot{M}$ eventually decreases back into the instability regime some decades after the nova outburst, then dwarf nova outbursts should resume. Both GK Persei [= Nova Per 1901; @Bian86] and V446 Herculis [= Nova Her 1960; @Hone98b] have displayed dwarf nova eruptions in their post-nova stages, and similar behavior has been suggested for a number of other old novae, commencing $\approx$ 50–200 yr after their outbursts [@Livi89; @Warn95 ch. 4]. The amplitudes of variability displayed by DI Lac and V841 Oph (see Figures \[f-DI\_lc\] and \[f-V8\_lc\]) are smaller than those typically observed for dwarf novae ($\Delta V \sim$ 2–4 mag), but we cannot rule out the possibility of a disk instability mechanism being in operation – after all, it has been 90 years since the nova outburst of DI Lac and over 150 years since that of V841 Oph. Further, the classic thermal instability mechanism is not well-explored for high-$\dot{M}$ disks, and other disk instabilities might operate as well [e.g. @Whit88; @Mine93; @Godo98].

The amplitudes of the $\sim30$–50 d oscillations in DI Lac and V841 Oph are similar to those of the spaced stunted outbursts seen in some novalike CVs [@Hone98]. One of the suggestions for stunted outbursts made in @Hone98 is that they are dwarf nova eruptions seen against a background of brighter light in the system. If that suggestion is correct, then the oscillations reported here [as well as similar 25 d oscillations reported for RW Tri, @Hone94] may be examples of dwarf nova type outburst behavior at relatively short recurrence times, $T_{\rm n}\lesssim50$ d [$T_{\rm n}$ is typically $\gtrsim10$ d to hundreds of days – even tens of years in extreme cases – for dwarf novae; @Warn95 ch. 3]. Among the true dwarf novae, the Z Camelopardalis stars have outburst recurrence times of $\approx10$–30 d, somewhat shorter than the time scales for the oscillations observed in these novae. The Z Cam stars are thought to have high mass transfer rates (near $\dot{M}_{\rm crit}$). This suggests that if the oscillations observed in novae are analogous to dwarf nova outbursts, then they may be linked to the presence of relatively high mass transfer rates that are, nonetheless, smaller than both $\dot{M}_{\rm crit}$ and $\dot{M}$ in the Z Cam stars.

Second, V841 Oph displays a distinct sinusoidal variation with a best period of 1800–1900 d (4.9–5.2 yr) or a slightly less preferred period of 1250–1300 d (3.4–3.6 yr). DI Lac does not appear to display similar sinusoidal variability, unless it occurs with a much longer period than in V841 Oph. @Bian90 reports a long-term period in V841 Oph of 3.4 yr with mean amplitude of $\sim0.3$ mag, which was determined from 420 visual observations spanning 30 yr [essentially the same early- to mid-20th century data utilized by @Duer92 see below]. @Rich94 re-analyzed the light curves presented by @Bian90 and consider the case for a multi-year periodicity in V841 Oph to be very weak. However, with our RoboScope light curve, sinusoidal variability on comparable multi-year times scales has now been observed in two separate data sets for V841 Oph spanning almost 80 years of observation. While this still does not provide firm evidence of strictly periodic behavior, it certainly points to the presence of a mechanism operating in this CV that modulates its brightness on a characteristic time scale of several years.

Photometric variability in old novae on time scales of several years has generally been attributed to solar-type magnetic cycles on the secondary star that might be able to control the rate of mass loss through the inner Lagrangian point [@Bian90; @Appl92]. Although @Rich94 find that no CV studied to date displays strictly periodic behavior over multi-year intervals, they also note that the observed amplitudes ($\sim0.2$ mag) and “apparent” time scales of variability of 5–40 yr are “plausible consequences from solar-type magnetic cycles.” We note that the shorter time scale ($\sim$30–50 d) variability discussed above might also be linked to modulation of $\dot{M}$ due to secondary star magnetic activity. If solar-type magnetic cycles can affect $\dot{M}$ on time scales of years, then starspots induced by the magnetic cycle that migrate under the $L_{1}$ point might also affect $\dot{M}$ on shorter time scales, in a manner suggested by @Livi94 to explain the very low brightness states seen in some CVs.

Third, the mean magnitudes of both CVs are changing slowly, at rates measured in a few millimagnitudes (mmag) per year. The mean brightness of DI Lac is increasing by 4 mmag yr$^{-1}$ since 1990. @Duer92 analyzed a large set of visual, photoelectric, and CCD observations of post-novae available in the literature [see references in @Duer92]. These data cover a large part of the 20th century, from the 1920’s through the 1980’s. @Duer92 summarizes the long-term behavior of DI Lac as exhibiting a steady decline of $13\pm1$ mmag yr$^{-1}$ in visual observations obtained between 1921 and 1952. Sparse photoelectric and CCD data from 1953 to 1981 suggest a possible small brightness increase, which is supported by our recent RoboScope observations. For V841 Oph, visual observations from 1919–52 and 1978–91 yield a brightness decline of $7\pm1$ mmag yr$^{-1}$, while “no definitive brightness decline” [@Duer92] was found in photoelectric data from 1954 to 1988. Our post-1991 observations of V841 Oph suggest a trend towards decreasing mean brightness comparable to that noted by @Duer92 in the archival visual observations, although the obvious sinusoidal variation that dominates our light curve at long time scales makes this conclusion somewhat suspect.

In the “classical” hibernation scenario [@Sh89], novae stay bright for $\sim$ 50–300 yr following outburst, gradually declining in brightness during this time to a hibernating state characterized by low brightness ($M_{V} \sim 10$ or fainter) and low mass accretion rate ($\dot{M} \lesssim 10^{-12}$ M$_{\odot}$ yr$^{-1}$). However, the photometric record of DI Lac from the 1920’s (a decade after its outburst) to the 1990’s does not show a simple decline in brightness. Instead, the brightness of DI Lac decreased for about four decades after outburst, but then apparently leveled off for several decades. In the last decade (covered in our RoboScope light curve), the brightness of DI Lac has been increasing. It is possible that we are seeing the influence of a mechanism (such as magnetic activity of the secondary star – see above – and/or accretion-induced irradiation of the secondary star) that modulates $\dot{M}$ (and, hence, the brightness) in DI Lac over a long-term cycle (with period on the order of many decades) that obscures the general decline predicted by the hibernation scenario.

Optical Spectrum Variability
----------------------------

The optical spectra of DI Lac and V841 Oph are quite different, and this can possibly be ascribed to their difference in post-outburst ages. The spectrum of V841 Oph is similar to those of novalike CVs [e.g. @Warn95 ch. 4] – this suggests that V841 Oph still has a high accretion rate, even $\approx150$ yr after outburst. The narrow emission components in DI Lac are suggestive of emission originating from the irradiated inner face of the secondary star, while the absorption troughs imply the presence of optically thick material in the system. The latter feature could be material ejected during the outburst of this younger post-nova; the lack of any detected H$\alpha$ emission shell [@Cohe85] does not preclude the existence of denser, non-emitting circumstellar material. The former feature offers a possible explanation for the gradual brightness increase seen in our RoboScope light curve of DI Lac (discussed in §\[s-optvar\]) if the secondary star is being slowly heated via irradiation and is, in turn, increasing the rate of mass transfer through the $L_{1}$ point.

Although the RoboScope coverage is incomplete, we infer from the adjacent data that DI Lac was faint during our July spectra and bright during our August spectrum (see Figure \[f-DI\_Xray\]). As mentioned in §\[s-optspec\], the only difference between our July and August spectra is that the emission cores were somewhat stronger in August (bright state) than July (faint state). This is consistent with the hypothesis that these narrow emission features originate on the irradiated face of the secondary star if we make the logical assumption that the irradiation increases when DI Lac is bright. It does not, however, illuminate the exact mechanism producing the irradiation (i.e. whether the “excess” flux in the high state originates near the disk center – presumably due to a disk instability producing increased accretion onto the WD – or in the outer disk – presumably due to an increase in mass transfer through the $L_{1}$ point.) If we assume that the spectrum shown in Figure \[f-DI\_spec\] (from July 27 09:01:47 UT) was obtained at an arbitrary orbital phase of 0.0, then the August 17 spectrum was obtained at a relative orbital phase of 0.9 [using $P_{\rm orb} = 0.543773$ d; @Ritt98]. Thus, these two spectra were obtained at similar orbital phases, and we do not expect the difference in the narrow emission component strength to be only due to system orientation.

The RoboScope coverage of V841 Oph during our spectroscopic (and X-ray) observations is more sparse than for DI Lac, but we can infer that the CV was returning to the faint state during our May 07 spectrum, was in its faint state during our May 14 and 17 spectra, and was likely near the bright state during our May 27 spectrum (see Figure \[f-V8\_Xray\]). (The two elevated brightness points at HJD 2450610 and HJD 2450625 suggest that V841 Oph may have returned to a bright state sometime between the low states bracketing HJD 2450590 and HJD 2450630.) If this is the case, then the somewhat stronger H$\alpha$ emission of the May 07 and May 27 spectra (see Figure \[f-V8\_spec\]) may be linked to the bright state of the CV. As with DI Lac, we calculated relative orbital phases for each of these spectra [using $P_{\rm orb} = 0.60423$ d; @Ritt98], and obtained $\phi = 0.0, 0.6, 0.75, 0.1$ for May 07, 14, 17, 27, respectively. Unfortunately, this casts some doubt on the link between H$\alpha$ emission strength and brightness state, since both of the bright state spectra (phases 0.0, 0.1) were obtained at different orbital phases than the faint state spectra (phases 0.6, 0.75). So, we cannot rule out the influence of system orientation effects in these spectra.

X-ray Variability
-----------------

Unfortunately, because of the weak X-ray emission from these CVs, little can be firmly stated about their X-ray variability. The count rates during visits 5 and 6 to DI Lac, which took place when we infer from the RoboScope light curve that the CV was in a bright state, are slightly elevated compared to the preceding visits. However, visit 7 also occurred during this optical bright state and does not show an elevated count rate. The mean X-ray count rate during each visit to V841 Oph also does not display any strong correlation with the corresponding optical state. The lack of large changes in X-ray flux or spectrum during the optical variations [as usually evident in dwarf novae; e.g. @Szko99], argues against a disk instability scenario. Alternatively, this could indicate that the optical brightness is determined by activity in the outer disk only and, therefore, is not reflected in the X-ray behavior; however, the X-ray count rates are too low (and their corresponding error bars too large) to make any firm conclusions.

Conclusions
===========

Our long-term optical light curves of the novae DI Lac and V841 Oph obtained with RoboScope reveal quasiperiodic variability with a characteristic time scale of $\sim30$–50 d in both CVs. In addition, the light curve of V841 Oph displays evidence for sinusoidal variability with a period of 3.5–5 yr. The latter cannot be said to be strictly periodic since our data set covers only $\approx1.5$ cycles; however, when this detection is added to past reports of multi-year periodicities in V841 Oph that have been reported in the literature, a strong case can be made for the presence of repeating multi-year variability with a preferred time scale in this CV. The most likely origin of such behavior is a solar-type magnetic cycle of the secondary star that modulates the rate of mass transfer through the $L_{1}$ point. If this is the case, then the shorter $\sim30$–50 d quasiperiodic variability in this system might also be related to the magnetic activity on the secondary star; for example, due to starspots that migrate under the $L_{1}$ point and temporarily throttle mass transfer. DI Lac does not show evidence for cyclic multi-year variability, unless it occurs with a much longer period than the length of our RoboScope coverage ($\gtrsim10$ yr). This casts some doubt on the origin of the $\sim30$–50 d quasiperiodic variability as a facet of the secondary star’s magnetic activity, since both V841 Oph and DI Lac show the $\sim30$–50 d quasiperiodic behavior, but only V841 Oph displays evidence for a multi-year solar-type magnetic cycle on the secondary star. On the other hand, the $\sim30$–50 d time scale is very reminiscent of that expected for dwarf nova type behavior (although the amplitude of variability in these novae is smaller than in typical dwarf nova outbursts). This raises the possibility that this variability is caused by a disk instability (either the thermal disk instability that leads to dwarf nova outbursts operating at a low level or some other form of disk instability).

The X-ray spectrum of DI Lac is fit almost equally well by the three models we tried: a simple blackbody, a Raymond-Smith thermal plasma, and bremsstrahlung emission. More complicated models (e.g. involving multiple components, lines, etc.) are unwarranted due to the low X-ray flux. In addition to being the most physically plausible X-ray emission emission mechanism in a (non-magnetic) CV, the bremsstrahlung model is formally the best-fitting. Our X-ray spectrum model fit parameters ($kT\sim4$ keV) are consistent with those obtained from $ROSAT$ X-ray spectra of a sample of 37 disk-accreting CVs [@Rich96]. Unfortunately, the X-ray count rates in both of these systems were too low for any conclusions to be made about their time-resolved X-ray behavior. X-ray observations sample the innermost disk region, and we would expect to see different time-resolved behavior as the novae go into their optically bright state if the transition is triggered by a disk instability vs. a change in $\dot{M}$ from the secondary star. Additional time-resolved X-ray observations using the more sensitive [*Chandra*]{} and/or [*XMM*]{} X-ray satellites may be necessary to illuminate the inner workings of these old novae.

PS, DWH, and VD acknowledge support from NASA grant NAG5-4791. DWH thanks the NOAO librarian Mary Guerrieri for her valuable assistance locating several papers cited herein. This research made use of NASA’s Astrophysics Data System Abstract Service and the SIMBAD database operated by CDS, Strasbourg, France.

[ ]{}

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[llcccl]{} 1990 Nov 12 & 2448207.7 & RoboScope & P & 240 & starting date\
1997 Jul 25 10:43:44 & 2450654.94822 & RXTE & X & 2528 & ID 20037-02-01-00\
1997 Jul 27 09:01:47 & 2450654.87744 & APO & S & 900 &\
1997 Jul 27 09:16:53 & 2450654.88792 & APO & S & 737 &\
1997 Jul 30 04:16:00 & 2450659.67921 & RXTE & X & 944 & ID 20037-02-02-00\
1997 Aug 03 04:11:12 & 2450663.68424 & RXTE & X & 1536 & ID 20037-02-03-00\
1997 Aug 07 04:12:00 & 2450667.67683 & RXTE & X & 1680 & ID 20037-02-04-00\
1997 Aug 12 03:42:40 & 2450672.65668 & RXTE & X & 592 & ID 20037-02-05-00\
1997 Aug 17 03:42:18 & 2450677.65666 & APO & S & 900 &\
1997 Aug 17 03:45:36 & 2450677.65893 & RXTE & X & 1024 & ID 20037-02-06-00\
1997 Aug 23 03:46:40 & 2450683.65991 & RXTE & X & 832 & ID 20037-02-07-00\
1997 Aug 27 03:46:56 & 2450687.66024 & RXTE & X & 736 & ID 20037-02-08-00\
1997 Sep 01 08:12:48 & 2450692.84503 & RXTE & X & 1616 & ID 20037-02-09-00\
1997 Sep 03 05:56:28 & 2450694.74974 & WIYN & S & 900 & No flux calibration\
1997 Sep 04 04:33:17 & 2450695.69269 & WIYN & S & 900 & No flux calibration\
1997 Sep 06 08:08:48 & 2450697.84239 & RXTE & X & 1776 & ID 20037-02-10-00\
1998 Nov 30 & 2451147.6 & RoboScope & P & 240 & ending date

[llcccl]{} 1991 May 31 & 2448407.7 & RoboScope & P & 240 & starting date\
1997 Apr 30 16:10:57 & 2450569.17897 & RXTE & X & 1936 & ID 20037-01-01-00\
1997 May 05 12:50:25 & 2450574.03997 & RXTE & X & 1856 & ID 20037-01-02-00\
1997 May 07 06:30:32 & 2450575.77120 & APO & S & 600 &\
1997 May 10 20:59:13 & 2450579.37966 & RXTE & X & 1936 & ID 20037-01-03-00\
1997 May 14 06:37:42 & 2450582.77618 & APO & S & 600 &\
1997 May 15 21:12:33 & 2450584.38911 & RXTE & X & 1408 & ID 20037-01-04-00\
1997 May 17 09:35 & 2450585.899 & WIYN & S & 4500 & 3 combined spectra\
1997 May 20 07:58:41 & 2450588.83795 & RXTE & X & 1792 & ID 20037-01-05-00\
1997 May 26 06:28:11 & 2450594.77523 & RXTE & X & 1872 & ID 20037-01-06-00\
1997 May 27 06:22:21 & 2450595.76552 & APO & S & 600 &\
1997 May 30 05:06:57 & 2450598.71888 & RXTE & X & 1648 & ID 20037-01-07-00\
1997 Jun 04 22:57:37 & 2450604.46244 & RXTE & X & 832 & ID 20037-01-08-00\
1997 Jun 09 06:43:45 & 2450608.78613 & RXTE & X & 2160 & ID 20037-01-09-00\
1997 Jun 18 16:24:33 & 2450618.18935 & RXTE & X & 1376 & ID 20037-01-11-00\
1998 Sep 04 & 2451060.5 & RoboScope & P & 240 & ending date

[ccc]{} 1 & 40 & 35\
2 & 29 & 36\
3 & 39 & 69\
4 & 31 & 40\
5 & 38 & 42\
6 & 75,40 & 49\
7 & 80,28 & 54\
8 & 31,21 & 48\
combined & 43,37 & 49,36

[lcccccc]{} Blackbody & 0.705 & 0.937 & $\le1.2$ & $2.2\times10^{-12}$ & $1.1^{+0.1}_{-0.2}$ & $3.0^{+0.6}_{-0.2}\times10^{-5}$\
Raymond-Smith & 0.617 & 0.982 & $\le3.3$ & $2.6\times10^{-12}$ & $3.2^{+0.7}_{-1.1}$ & $3.4^{+3.5}_{-0.6}\times10^{-3}$\
Bremsstrahlung & 0.484 & 0.999 & $\le1.8$ & $2.7\times10^{-12}$ & $4.5^{+1.3}_{-1.3}$ & $1.1^{+0.7}_{-1.1}\times10^{-3}$\

Blackbody & 0.683 & 0.954 & 0.09 & $2.2\times10^{-12}$ & $1.1^{+0.1}_{-0.1}$ & $3.1^{+0.4}_{-0.3}\times10^{-5}$\
Raymond-Smith & 0.604 & 0.986 & 0.09 & $2.7\times10^{-12}$ & $2.9^{+0.9}_{-0.5}$ & $3.7^{+0.9}_{-0.9}\times10^{-3}$\
Bremsstrahlung & 0.471 & 0.999 & 0.09 & $2.7\times10^{-12}$ & $4.2^{+1.5}_{-1.0}$ & $1.2^{+0.4}_{-0.3}\times10^{-3}$\

Blackbody & 0.689 & 0.951 & 0.24 & $2.3\times10^{-12}$ & $1.1^{+0.1}_{-0.1}$ & $3.3^{+0.3}_{-0.4}\times10^{-5}$\
Raymond-Smith & 0.606 & 0.986 & 0.24 & $2.7\times10^{-12}$ & $2.9^{+0.9}_{-0.5}$ & $3.8^{+0.9}_{-0.9}\times10^{-3}$\
Bremsstrahlung & 0.475 & 0.999 & 0.24 & $2.7\times10^{-12}$ & $4.1^{+1.4}_{-1.0}$ & $1.2^{+0.4}_{-0.3}\times10^{-3}$

[^1]: also see <http://www.apo.nmsu.edu/>

[^2]: see <http://www.noao.edu/wiyn/>

[^3]: also see <http://heasarc.gsfc.nasa.gov/docs/xte/>

[^4]: <http://heasarc.gsfc.nasa.gov/docs/xte/recipes/cook_book.html>

[^5]: This routine iteratively refits the model to the data while stepping the values of selected parameters through a given range and produces a plot in parameter space of the 2-dimensional $\chi^{2}$ contours.

[^6]: Visits 8 and 10 were not used. The former was deemed to be an “intermediate” brightness state, and the latter an anomalously low state.
