An extensive run of photometric observations of HDE 226868 is presented as a constraint on theoretical models of the Cygnus X-1 system. A period of about 5.6015 days is found to be the best fit to the data for all 349 nights of observation, and the light curve obtained by folding the data with this period is shown to exhibit an overall double-peaked variation with an amplitude of 0.045 magnitude as well as short-duration features near both minima and both maxima. Changes in the form of the light curve and in the brightness of HDE 226868 are discussed, and it is suggested that the light curve has two extreme forms whose combination results in many of the details of the five-year mean light curve. Temporary increases in the X-ray flux from Cyg X-1 are described, results of X-ray photometry are plotted against Julian day, and the optical brightness variations are tentatively attributed to continuum-level changes. Observations with which any future model of the Cyg X-1 system must be compatible are briefly summarized.

Peer reviewed