The quiescent light curve and orbital period of GRO~J0422+32

Abstract

CCD photometry of the black hole candidate GRO~J0422+32 in quiescence, obtained at Haute-Provence from 1994 November to 1995 February, reveals a double-wave modulation at a period close to the value we found during outbursts and also close to one of the possible periods derived by Filippenko, Matheson and Ho (1995) from spectroscopic observations with the W. M. Keck 10-m telescope. A period of $0.212140 \pm 0.000003$ d ($5.09136 \pm 0.00007$ h) fits all our photometric data from 1993 January to 1995 February and yields a minimum in our light curves at the inferior conjunction of the M star, as determined from the radial velocity data of Filippenko et al. (1995). The quiescent $R_{c}$ band light curve exhibits a changing asymmetry of shape and a variable amplitude. On two consecutive nights the source was found constant to within $ \pm 0.05$ mag, suggesting an upper limit on the ellipsoidal effect in this band.