  We demonstrate correlated rapid variability between the optical/UV and X-ray
emission for the first time in a soft X-ray transient, GRO J1655-40: HST light
curves show similar features to those seen by RXTE, but with mean delay of
10-20s. We interpret the correlations as due to reprocessing of X-rays into
optical and UV emission, with a delay due to finite light travel time, and thus
perform echo mapping of the system. The time-delay distribution has a mean of
14.6s +/- 1.4 and dispersion (i.e. the standard deviation of the distribution)
of 10.5s +/- 1.9 at binary phase 0.4. Hence we identify the reprocessing region
as the accretion disc rather than the mass donor star.
