THE QUIESCENT LIGHT-CURVE AND ORBITAL PERIOD OF GRO J0422+32

CCD photometry of the black hole candidate GRO J0422+32 in quiescence, obtained at Haute-Provence from 1994 November to 1995 February, revea ls a double-wave modulation at a period close to the value we found du ring outbursts and also close to one of the possible periods derived b y Filippenko, Matheson and Ho (1995) from spectroscopic observations w ith the W. M. Keck 10-m telescope. A period of 0.212140+/-0.000003 d ( 5.09136+/-0.00007 h) fits all our photometric data from 1993 January t o 1995 February and yields a minimum in our light curves at the inferi or conjunction of the M star, as determined from the radial velocity d ata of Filippenko et al. (1995). The quiescent R(c) band light curve e xhibits a changing asymmetry of shape and a variable amplitude. On two consecutive nights the source was found constant to within +/-0.05 ma g, suggesting an upper limit on the ellipsoidal effect in this band.