  We have used the COBE DIRBE database to derive near- and mid-infrared light
curves for a well-defined sample of 38 infrared-bright Mira variable stars, and
compared with optical data from the AAVSO. In general, the 3.5 micron and 4.9
micron DIRBE bandpasses provide the best S/N light curves, with S/N decreasing
with wavelength at longer wavelengths. At 25 microns, good light curves are
only available for ~10 percent of our stars, and at wavelengths >= 60 microns,
extracting high quality light curves is not possible. The amplitude of
variability is typically less in the near-infrared than in the optical, and
less in the mid-infrared than in the near-infrared, with decreasing amplitude
with increasing wavelength. On average, there are 0.20 +/- 0.01 magnitudes
variation at 1.25 microns and 0.14 +/- 0.01 magnitudes variation at 4.9 micron
for each magnitude variation in V. The observed amplitudes are consistent with
results of recent theoretical models of circumstellar dust shells around Mira
variables. For a few stars in our sample, we find clear evidence of time lags
between the optical and maxima of phase ~ 0.05 - 0.13, with no lags in the
minima. For three stars, mid-infrared maximum appears to occur slightly before
that in the near-infrared,but after optical maximum. We find three examples of
secondary maxima in the rising portions of the DIRBE light curves, all of which
have optical counterparts in the AAVSO data, supporting the hypothesis that
they are due to shocks rather than newly-formed dust layers. We find no
conclusive evidence for rapid (hours to days) variations in the infrared
brightnesses of these stars.
