ORBITAL PERIOD CHANGES OF CYGNUS-X-3

We describe the orbital period change of the X-ray binary Cygnus X-3, and present three new times of X-ray minima, obtained using the Ginga and ASCA satellites. Before 1990, the long-term ephemeris of Cyg X-3 r equired the introduction of a second time derivative, P, of order -(1. 27 +/- 0.19) x 10(-10) yr(-1) This value indicates that the first time derivative, P, changes from a positive to a negative value around 198 6. However, the new ASCA data points presented here do not confirm thi s large P variation and indicate a significantly smaller P of -(8.13 /- 1.15) x 10(-11) yr(-1) and a global period change of P/P = (1.17+/- 0.44) x 10(-6) yr(-1). The derived P can be explained by a large mass- loss rate of 0.58 x 10(-6) M(T) yr(-1), where M(T) is the total mass o f the Cyg X-3 system. This value is consistent with the expected mass- loss rate of the Wolf-Rayet star of about 10(-6)M(.) yr(-1). The exist ence of a red-noise type fluctuation is also described.