Evidence for an approximatily 300 day period in Cygnus X-1.

Results from 10 years of observation of Cyg X-1 by the Vela 5B satellite are reported. Good evidence for an approximately 300 day period is found, which is confirmed by independent data from the All-Sky Monitor instrument on Ariel 5. Cyg X-1 varies by about 25 percent with a 294 + or -4 day period. This modulation is apparently unrelated to the known transitions between the source high and low states. Flux minima occur at 1974.05+nP. The 294 day period is consistent with the precession of the supergiant companion HDE 226868 and also with the precession period of a tilted accretion disk. The light curve could be modulated by a change in the mass transfer rate or variable obscuration by ionized matter.