  We address the task of modeling soft X-ray and optical light curves of X-ray
novae in the high/soft state. The analytic model of viscous evolution of an
externally truncated accretion \alpha-disk is used. Relativistic effects near a
Kerr black hole and self-irradiation of an accretion disk are taken into
account. The model is applied to the outbursts of X-ray nova Monocerotis 1975
(A 0620-00) and X-ray nova Muscae 1991 (GRS 1124-68). Comparison of
observational data with the model yields constraints on the angular momentum
(the Kerr parameter) of the black holes in A 0620-00 and GRS 1124-68: 0.3-0.6
and \leq 0.4, and on the viscosity parameter \alpha of the disks: 0.7-0.95 and
0.55-0.75. We also conclude that the accretion disks should have an effective
geometrical thickness 1.5-2 times greater than the theoretical value of the
distance between the photometric layers.
