The Orbital Ephemeris and X-Ray Light Curve of Cyg X-3

The orbital dynamics of Cyg X-3 are a key to understanding this enigmatic X-ray binary. Recent observations by the RXTE ASM and the OSSE instrument on GRO enable us to extend the baseline of arrival time measurements and test earlier models of orbital period evolution. We derive new quadratic and cubic ephemerides from the soft X-ray data (including ASM). We find a significant shift between the predicted soft X-ray phase and the light curve phase measured by OSSE from ~44 to 130 keV. Some of the apparent phase shift may be caused by a difference in light curve shape.