ИСТИНА

We continue our study of spectral and photometric variability of Cyg X-1 on the base of the 12-year-long series of precision spectral observations and 40-year long series of multicolor photometric observations we have accumulated up to now. In 2006, we revealed for the first time that the temperature of the optical component (О9.7Iab supergiant) was decreasing and its size was increasing over 7 years (1997–2003). Photometry performed at the Crimean Station of the Sternberg Astronomical Institute (Lomonosov Moscow State University) shows that the supergiant’s variability on the time scale of decades continues up to now.