The 4.8 hour modulation of Cyg X-3.

The relatively high rate of change of the 4.8-hr modulation period of Cyg X-3 reported in the literature period derivative approximately 5 x 10 to the -6th/year is not confirmed by X-ray observations taken with the Copernicus satellite over an interval of seven years. Using the Copernicus data, we find a three-standard-deviation upper limit to the period derivative of 7 x 10 to the -7th/year between 1972 and 1978. This result removes the bias towards high-mass-loss binary models of Cyg X-3 implied by the previously reported positive detections of a period derivative. However, accretion onto a compact star remains a viable explanation for the origin of the X-ray flux.