ИСТИНА

Here we report the results of UBV photometry and H-alpha spectroscopy of the X-ray binary, the well-known black hole candidate Cyg X-1/V1357 Cyg during an X-ray burst in May-October 1996. The observations were carried out with the aim to follow the photometric behaviour of optical star in this system during the X-ray burst. We also try to find the possible correlation between the long-term photometric and X-ray variations. The comparison of Cyg X-1 light curves obtained from our UBV-observations with the mean light curves for the preceding years clearly shows the increase of the average level of Cyg X-1 light curve by ~0^m.03 in B-band and by ~0^m.05 in U-band. The average level in V-band is nearly the same. The most interesting result is that the shape of Cyg X-1 optical light curve varied dramatically during the period of our observations. At the maximum of X-ray burst (X-ray source ahead) it can be represented by a single wave with one maximum, - the secondary maximum is absent. In August by the end of the X-ray burst the additional radiation in the primary minimum is observed (when supergiant is ahead). And then, after the return to the X-ray low state (September-November), the shape of the orbital light curve come to the normal appearence of ellipsoidal curve. The shape changes were also followed by the significant variations of line profile and intensity of H-alpha emission: the H-alpha intensity during the outburst maximum was dumped. The mean for the year U,B,V magnitudes distinctly show that there is a secular increase of supergiant luminosity during the period 1971-1996 with the possible periodic component superimposed, which nature is quite different from the nature of secular increase. The anomal high power of X-ray burst in 1996 possibly is due to increase of supergiant luminosity in this system.