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 HJD 245 0235.29+n×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 such as that suggested by Ninkov, Walker & Yang. We discuss the shortcomings of previous work in establishing the period and orbital elements.
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LaSala et al. (1998) studied this question.