Аннотация:We study descending light-curve branches of X-ray Novae in the model of non-stationary disk accretion on relativistic stars. At the first accretion stage, hydrogen is completely ionized in the whole disk, and the disk size is constant. After some 30-70 days after the X-ray luminosity maximum of the disk, a zone is formed at its outer edge where hydrogen is partially ionized. Convection in the direction perpendicular to the disk radius appears in this zone, which effectively increases viscosity and the accretion rate; the light curve exhibits a secondary peak. At this stage, the boundary between the zone of completely ionized hydrogen and the zone of neutral hydrogen begins to move inward, along the disk radius. The character of decline on the light curve changes. Our computations explain observations satisfactorily.