The nature of optical variability of the CygX2=V1341Cyg and ScoX1=V818Sco X-ray binaries

The results of observations of Cyg X2 and SCO X1 over the 1974-1975 period are presented. A fast irregular variability of the Cyg X2 optical radiation has been revealed. The upper limit of the periodic component half-amplitude in the brightness curve of this object is (<=) 0sup(m)x1 at the total amplitude of irregular oscillations of brightness reaching 1sup(m). Such a small amplitude of the periodic component may be attributed to a considerable contribution to the optical brightness of the system made by the accretion disc around an X ray source. The radiation spectrum of the disc has been calculated and its contribution to the brightness has been shown to exceed that of the ''hot spot'' on the surface of a normal star heated by X rays and responsible for the periodic component of the brightness curve