The period derivative of Cygnus X-3

X-rays from Cygnus X-3 have been observed during early 1978 with the detectors of the SAS 3 satellite. These observations, in conjunction with earlier Uhuru and ANS data, indicate that the 4.8 hr period of Cygnus X-3 is increasing at a rate of (5.1 + or - 1.3) times to the -6th per year. The sign and magnitude for this change are incompatible with a rotation model for the period and are in reasonable agreement with model predictions for orbital changes associated with mass loss and transfer in a binary system.