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 $$(\dot{P}/P\sim 5 \times {10}^{-6}\,\text{yr}^{-1})$$ 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 3 a upper limit to $$\dot{P}/P\,\text{of} \,7 \times {10}^{-7}\text{yr}^{-1}$$ 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.