Cygnus X-1 : optical variation on the 294 day X-ray period.

The recently discovered 294 day X-ray period has optical counterparts. A 10 yr B band photometric record shows synchronous changes of 0.01 mag amplitude, with minima coinciding with the X-ray minima. Stronger and weaker variations, respectively, are seen in U and V. The 5.6 day orbital light curves, additionally, show pronounced changes on the 294 day cycle. There is also a 294 day polarization variation.