  UCAC2 46706450 is a late-type star with an ultraviolet (UV) excess. It was
considered a candidate to establish a sample of FGK stars with white dwarf (WD)
companions that can be used to test binary evolution models. To verify the WD
nature of the companion, UV spectroscopy was performed by Parsons et al.
(2016). By a detailed model-atmosphere analysis we show that the UV source is
an extremely hot WD with effective temperature $T_\mathrm{eff}$ =
$105\,000\pm5000$ K, mass $M/M_\odot = 0.54\pm0.02$, radius $R/R_\odot =
0.040^{+0.005}_{-0.004}$, and luminosity $L/L_\odot= 176^{+55}_{-49}$, i.e.,
the compact object is just about to enter the WD cooling sequence.
Investigating spectra of the cool star ($T_\mathrm{eff}$ = $4945\pm250$ K) we
found that it is a K-type subgiant with $M/M_\odot = 0.8-2.4$, $R/R_\odot =
5.9^{+0.7}_{-0.5}$, and $L/L_\odot= 19^{+5}_{-5}$, that is rapidly rotating
with $v \sin(i)=81$ km s$^{-1}$. Optical light curves reveal a period of two
days and an o-band peak-to-peak amplitude of 0.06 mag. We suggest, that it is
caused by stellar rotation in connection with star spots. With the radius we
infer an extremely high rotational velocity of
$v_{\mathrm{rot}}=151^{+18}_{-13}$ km s$^{-1}$, thus marking the star as one of
the most rapidly rotating subgiants known. This explains chromospheric activity
observed by H$\alpha$ emission and emission-line cores in CaII H and K as well
as NUV flux excess. From equal and constant radial velocities of the WD and the
K subgiant as well as from a fit to the spectral energy distribution we infer
that they form a physical, wide though unresolved binary system. Both
components exhibit similar metal abundances and show iron-group elements with
slightly oversolar (up to 0.6 dex) abundance, meaning that atomic diffusion in
the WD atmosphere is not yet active due to a residual, weak radiation-driven
wind. (abridged)
