PHOTOMETRIC JK OBSERVATIONS OF CYG X-1 - BINARY PARAMETERS FROM OPTICAL AND INFRARED DATA

Photometric JK observations of the X-ray binary system Cyg X-1 were ca rried out over a period of ten years (1984-1994). The J and K light cu rves obtained for Cyg X-1 can be described by those binary parameters which have been previously derived from analysis of the highly accurat e optical B light curve of Cyg X-1, but only if the degree of Roche-lo be filling in the infrared is assumed to be close to unity (mu = 1.0); the preferred value in the optical is mu similar to 0.9. The infrared light curves are consistent with the model of ellipsoidal variability of Cyg X-1. The J and K light curves have a considerably smaller ampl itude than the published ones. The reason for this difference is not y et clear; further infrared observations of Cyg X-1 are needed to clari fy it.