On the mass of the compact object in GRO J0422 + 32

We report the detection of the secondary star spectrum in the hard X-ray transient GRO J0422 + 32 during its lowest quiescent state. From the amplitude of the radial velocity curve we derive a mass function f(M) = 0.85 +/- 0.30 solar mass. We classify the companion star as type M2 +/- 2 V which contributes 35 +/- 6 per cent of the total flux in the spectral range 6000-6500 A and 52 + /- 8 per cent in the range 6700-7500 A. Ellipsoidal modulation fits to the I-band light curves constrain the inclination to be between 41 deg +/- 6 deg and 43 deg +/- 6 deg for mass ratios q = 10 exp -5. We show that the primary is likely to be a low-mass black hole with a probable mass of about 2.5-5 solar masses. (Author)