2000MNRAS.317..528W

We present 53 simultaneous photometric (I band) and spectroscopic (6900-9500Å) observations of GRO J0422+32, taken during 1997 December. From these we determine that J0422+32 was in its lowest state yet observed, at I=20.44±0.08. Using relative spectrophotometry, we show that it is possible to correct very accurately for telluric absorption. Following this, we use the TiO bands at 7055 and 7589Å for a radial velocity study and thereby obtain a semi-amplitude of 378±16km.s^–1, which yields f(M)=1.191±0.021M_☉ and {formmu1} consistent with previous observations. We further demonstrate that this little-explored method is very powerful for such systems. We also determine a new orbital ephemeris of HJD=2450274.4156±0.0009+0.2121600±0.0000002E.

We see some evidence for an ellipsoidal modulation, from which we determine the orbital inclination of J0422+32 to be less than 45°. We therefore calculate a minimum mass for the primary of 2.22M_☉, consistent with a black hole, but not necessarily the supermassive one proposed recently (1997) by Beekman et al. We obtain an M4-5 spectral type for the secondary star, and determine that the secondary contributes 38±2per cent of the flux that we observe from J0422+32 over the range 6950-8400Å. From this we calculate the distance to the system to be 1.39±0.15kpc.