ИСТИНА

The photoelectric UBVR and WBVR observations of V1357 Cyg (Cyg X-1) were carried out at Tien-Shan and Mt. Maidanak observatories from October 1995 to November 1996 during 115 nights (total of 827 observations). 102 of them are overlapping with ASM/RXTE data. In May-August 1996, during our observation set, the object was in high soft X-ray state, and from August 9 it returned to its ordinary hard state. The accuracy of our photometry varies in the range of 0.003-0.006 mag on different nights. The mean orbital light curve fits well the calculated one in the model of the tidally distorted star. The corrected orbital period is 5.599763 +/- 0.000020 day. We found that the mean brightness level was changing during the high soft state. It was lower by 0.014 mag than normal in the beginning and reached the normal one to the end of this state. The optical light curve shows flares (0.025 mag) lasting from few hours to few days and dips (0.01 mag), in most cases coinciding with X-ray ones; the flares (0.04 mag) lasting several days coinciding with X-ray dips. So the orbital light curve during the soft state conserves its two-wave shape, sometimes being distorted with these features. In the early November 1996, we observed an outstanding deep fading of the optical counterpart by 0^m.04 in all the bands which lasted for about a week. The amplitude of fading was equal to the earlier estimated contribution of the accretion disc to the common light of the system. This fading occured between two flaring episodes in the soft X-ray and roughly coincided with the 20 percent dip. We performed Fourier analysis of our optical and ASM/RXTE X-ray data and revealed the same frequencies in optical and X-ray observational data (in optical data: 100, 17, 50 days; in X-ray: 50, 8.6, 17 days), besides the orbital period. The 50 day period corresponds to the flaring episodes described by Zhang et al. (IAUC 6462,1996). Several frequencies are found both in optical and X-ray range. The orbital period was found in the X-ray data at the level of 7 per cent. We may conclude that there is some correspondence between optical and X-ray variations.