  We present a method of analysing the correlated X-ray and optical/UV
variability in X-ray binaries, using the observed time delays between the X-ray
driving lightcurves and their reprocessed optical echoes. This allows us to
determine the distribution of reprocessing sites within the binary. We model
the time-delay transfer functions by simulating the distribution of
reprocessing regions, using geometrical and binary parameters. We construct
best-fit time-delay transfer functions, showing the regions in the binary
responsible for the reprocessing of X-rays. We have applied this model to
observations of the Soft X-ray Transient, GRO j1655-40. We find the optical
variability lags the X-ray variability with a mean time delay of 19.3$pm{2.2}$
seconds. This means that the outer regions of the accretion disc are the
dominant reprocessing site in this system. On fitting the data to a simple
geometric model, we derive a best-fit disk half-opening angle of
13.5$^{+2.1}_{-2.8}$ degrees, which is similar to that observed after the
previous outburst by cite{orosz97}. This disk thickening has the effect of
almost entirely shielding the companion star from irradiation at this stage of
the outburst.
