  We present a multi-wavelength timing study of the eclipsing low mass X-ray
binary EXO 0748-676 (UY Vol) using XMM-Newton when the source was in a hard
spectral state. The orbital optical and X-ray light curves show a fairly large
amount of intensity modulation in the 7 observations taken during
September-November, 2003, covering 36 complete binary orbits of EXO 0748-676.
While assessing the non-burst variability, simultaneously in the optical and
X-ray light curves, we find that they are not correlated at reprocessing or
orbital time-scales, but are weakly correlated at a few 1000s of seconds
time-scales. Although a large fraction of the optical emission is likely to be
due to reprocessing, the lack of significant correlation and presence of large
variability in the orbital X-ray and optical light curves is probably due to
structures and structural changes in the accretion disk that produce, and
sometimes mask the reprocessed signal in varying amounts. These disk structures
could be induced, at least partly, by irradiation. From the observed
modulations seen in the optical light curves, there is strong evidence of
accretion disk evolution at time scales of a few hours.
