ORBITAL AND QUASI-PERIODIC OPTICAL VARIATIONS IN THE BLACK-HOLE X-RAYBINARY V404 CYG

We present 2 weeks of CCD photometry of V404 Cyg, the optical counterp art of the soft X-ray transient (SXT) GS 2023+338, taken from La Palma in 1992 July. The R-band light curve shows a clear 0.3-mag ellipsoida l modulation on the established 6.5-d orbital period, but with substan tial (up to 0.3 mag) superposed variations that occur during each nigh t on a time-scale of 6 h. We model both the mean orbital light curve a nd its lower envelope (which we presume to show the secondary star wit h minimum contamination from the accretion disc). With a mass ratio al ready known from the optical spectroscopy of Casares & Charles, the an alysis of the lower envelope light curve leads to an orbital inclinati on of i = 56 degrees +/- 2 degrees. Our best model requires no hotspot contribution. The shorter time-scale of variability is consistent wit h the 6-h quasi-periodicities that have been seen in outburst and duri ng the early quiescent period. Using periodogram analysis, we find thr ee components of the 6-h peak (at 5.90, 6.07 and 6.24 h). The periodog rams show the outside peaks to be stronger than the smaller, central p eak, A peak in the power spectrum at 4.5 d coincides with the beat per iod of these outside peaks. The nature of these periodicities is unkno wn.