The quiescent light curve and the evolutionary state of GRO J1655-40

We present ellipsoidal light-curve fits to the quiescent B, V, R and I light curves of GRO J1655-40 (Nova Scorpii 1994). The fits are based on a simple model consisting of a Roche-lobe-filling secondary and an accretion disc around the black hole primary. Unlike previous studies, no assumptions are made concerning the interstellar extinction or the distance to the source; instead these are determined self-consistently from the observed light curves. In order to obtain tighter limits on the model parameters, we used the distance determination from the kinematics of the radio jet as an additional constraint. We obtain a value for the extinction that is lower than was assumed previously; this leads to lower masses for both the black hole and the secondary star of 5.4 +/- 0.3 and 1.45 +/- 0.35 M-circle dot, respectively. The errors in the determination of the model parameters are dominated by systematic errors, in particular arising from uncertainties in the modelling of the disc structure and uncertainties in the atmosphere model for the chemically anomalous secondary in the system. A lower mass of the secondary naturally explains the transient nature of the system if it is in either a late case A or early case B mass-transfer phase.