The Orbital Period of HDE226868/Cyg X-1
  We present epoch 1996, high-quality radial velocity data for HDE 226868, the optical counterpart of Cygnus X-1. Combining our results with all published historical data, we have derived a new ephemeris for the system of HJD2450235.29 + n x 5.5998, which allows accurate orbital phase calculations to be made for any X-ray observations over the last 30 years. We find no evidence for any period change as has been suggested by Ninkov, Walker & Yang (1987). We discuss the shortcomings of previous work in establishing the period and orbital elements. 
J. LaSala(1,2), P.A. Charles(1), R.A.D. Smith(1), M.   Balucinska-Church(3) and M.J. Church(3) ((1)Department of Astrophysic...