Evidence for evolving accretion disc structure in 4U1957+115

We present results of the first multicolour photometric study of 4U1957+115, one of the optically less studied low-mass X-ray binaries (LMXBs). Our quasi-simultaneous UBVRI observations reveal that the light-curve pulse shape over the 9.33-h period discovered by Thorstensen has changed substantially since his earlier V-band studies. The light curve now shows a clear asymmetry and an amplitude more than double that seen by Thorstensen. The light curve also shows a colour dependence that seems to rule out the X-ray-heating model as an origin for photometric variability. We believe that the changes observed in the light-curve shape are indicative of evolving accretion disc structure. We discuss the implications of this for the origin of optical emission in 4U1957+115 and disc-accreting binaries in general.