Evidence for evolving accretion disk structure in 4U 1957+115
  We present results of the first multicolour photometric study of 4U1957+115, one of the optically less studied low mass X-ray binaries (LMXB). Our quasi-simultaneous UBVRI observations reveal that the light curve pulse shape over the 9.33 hr period discovered by Thorstensen (1987) has changed substantially since his earlier V band studies. The light curve now shows clear asymmetry and an amplitude more than double of that seen by Thorstensen (1987). The light curve also shows 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 disk structure. We discuss implications of this on the origin of optical emission in 4U1957+115 and disk accreting binaries in general. 
Pasi Hakala, Panu Muhli and Guillaume Dubus