Photometric observations of high mass X-ray binary HD 226868

The results of photometric observations of the high mass X-ray binary HD 226868 = Cyg X-1 have been presented.The observations of this star were performed at 60cm telescope of ShAO named after N.Tusi by using CCD photometer.This star is a binary system consisting of a black hole and a massive giant component star.Black hole and optical giant component masses are M = 20 5M and M opt = 40 5M , respectively.The blue supergiant orbiting the black hole is HDE 226868 and its spectral type is O9.7 Iab.Our aim was investigating variability of this star in the V filter of the BVRI photometric system.Together with our photometric observations we used also AAVSO photometric database for the star HD 226868.Short-term and long-term photometric variability of this star has been revealed.The low luminosity state of star HD 226868 was discovered by analyzing the AAVSO photometric database, where V magnitude changes in the interval between 9 m .1 and 9 m .2.The lower luminosity state was observed on November 16, 2021, from 19 h 55 m 12 s to 20 h 11 m 02 s .This state may be related to the supergiant wind instability as well as changes in the supergiant's mass transfer rate.