The quiescence optical light curve of Nova Scorpii 1994 ( = GROJ 1655 – 40 ) ⋆

We report on extensive V, R, and i band photometry of the black-hole candidate Nova Sco 1994 (GRO J1655–40) obtained during March 1996, when the source was close to its quiescent pre-outburst optical brightness (V=17.3). From our observations and data taken from the literature we derive a refined ephemeris for inferior conjunction of the secondary star: HJD 244 9838.4198(52) + 2.62168(14)×N. We have modeled the V, R, and i band light curves in terms of a Roche lobe filling secondary and an accretion disk around the compact star, the latter described as a flat cylinder with a radial temperature distribution. From the shape of the light curve we constrain the binary inclination and mass of the secondary star to lie in the ranges 63. ◦7–70. ◦7 and 1.60–3.10 M , respectively. This limits the mass of the black-hole to the range 6.29–7.60 M . The mass range we obtained for the secondary star is supported by the results of stellar evolution calculations.