  The UV emission from X-ray binaries, is more likely to be produced by
reprocessing of X-rays by the outer regions of an accretion disk. The structure
of the outer disk may be altered due to the presence of X-ray irradiation and
we discuss the physical regimes where this may occur and point out certain
X-ray binaries where this effect may be important. The long term X-ray
variability of these sources is believed to be due to stochastic fluctuations
in the outer disk, which propagate inwards giving rise to accretion rate
variation in the X-ray producing inner regions. The X-ray variability will
induce structural variations in the outer disk which in turn may affect the
inner accretion rate. To understand the qualitative behaviour of the disk in
such a scenario, we adopt simplistic assumptions that the disk is fully ionised
and is not warped. We develop and use a time dependent global hydrodynamical
code to study the effect of a sinusoidal accretion rate perturbation introduced
at a specific radius. The response of the disk, especially the inner accretion
rate, to such perturbations at different radii and with different time periods
is shown. While we didn't find any oscillatory or limit cycle behaviour, our
results show that irradiation enhances the X-ray variability at time-scales
corresponding to the viscous time-scales of the irradiated disk.
