An optical precursor to the recent soft X-ray outburst the black hole binary GRO J1655-40

We discuss the observation of an optical outburst that preceded the April 1996 X-ray outburst of GRO J1655-40 by approximate to 6 days. The onset of the optical brightening was earliest in I and latest in B, however the rate of increase was largest in B and smallest in I. The dependence of the onset times and rates of increase on the color strongly suggests an "outside-in" disturbance of the accretion disk. The substantial delay between the optical outburst and the X-ray outburst constrains models of the accretion flow in this system.