Observations of GRO J0422+32 .3. A low-inclination black hole X-ray nova

We present the results of our campaign of photometry of the X-ray nova GRO J0422+32 in quiescence, the purpose of which was to confirm the proposed orbital period of similar to 5.1 hr, and to search for the ellipsoidal variations which are a prerequisite for the mass determination of the primary. We find evidence for a weak ellipsoidal modulation corresponding to an orbital period of 5.1 hr, with a semiamplitude of similar to 0.03 mag-the lowest known of any quiescent X-ray nova. With the assumption that the I-band contamination by the accretion disk is no greater than that observed in other quiescent X-ray novae, we estimate that the orbital inclination must be less than or equal to 45 degrees. Furthermore, a ZAMS secondary implies an upper limit on the distance of similar to 2.2.kpc, and a quiescent accretion rate similar to 3% that observed in A0620-00. The inclination estimate implies a mass of the compact object greater than or equal to 3.4 M(.). ROSAT observations constrain the quiescent X-ray luminosity to be less than 2 X 10(32) ergs s(-1) (for a distance of 2 kpc). We speculate that the low quiescent accretion rate through the disk, inferred from the optical observations, may well be related to the large outburst amplitude of GRO J0422+32.