  Variation of mass supply rate from the companion can be smeared out by
viscous processes inside an accretion disk. By the time the flow reaches the
inner edge, the variation in X-rays needs not reflect the true variation of the
rate at the outer edge. However, if the viscosity fluctuates around a mean
value, one would expect the viscous time scale also to spread around a mean
value. In HMXBs, the size of the viscous Keplerian disk is smaller & thus such
a spread could be lower as compared to the LMXBs. If there is an increasing or
decreasing trend in viscosity, the interval between enhanced emission would be
modified systematically. In the absence of a full knowledge about the variation
of mass supply rates at the outer edge, we study ideal circumstances where
modulation must take place exactly in orbital time scales when there is an
ellipticity in the orbit. We study a few compact binaries using long term
RXTE/ASM(1.5-12 keV) & Swift/BAT(15-50keV) data to look for such effects & to
infer what these results can tell us about the viscous processes inside the
respective disks. We employ three different methods to seek imprints of
periodicity on the X-ray variation & found that in all the cases, the location
of the peak in the power density spectra is consistent with the orbital
frequencies. Interestingly, in HMXBs the peaks are sharp with high rms values,
consistent with a small Keplerian disk in a wind fed system. However, in LMXBs
with larger Keplerian disk, the peaks are spread out with much lower rms
values. X-ray reflections, or superhump phenomena which may also cause such
X-ray modulations would not be affected by the size of the Keplerian disk. Our
result confirms different sizes of Keplerian disks in 2 classes of binaries. If
the orbital period of a binary system is not known, it may be obtained with
reasonable accuracy for HMXBs & with lesser accuracy for LMXBs by our method.
