1999A&A...343..861B

By accumulating all available radial velocity data and combining it with our own spectroscopy we present a new and improved ephemeris for Cygnus X-1 (V1357 Cygni/HDE226868). We initially calculate an orbital period of 5.599847±0.000018 days and then, by including the more recent radial velocities of LaSala et al. (1998MNRAS.301..285L) and Sowers et al. (1998ApJ...506..424S) we refine this to 5.599829±0.000016 days. Using 27 years' worth of photometry, we also calculate a photometric period of 5.599836±0.000024 - remarkably similar to the spectroscopic one.