  We fit outbursts of two X-ray novae (Nova Monocerotis 1975=A0620-00 and Nova
Muscae GS 1991=1124-683) using a time-dependent accretion disk model. The model
is based on a new solution for a diffusion-type equation for the
non-steady-state accretion and describes the evolution of a viscous alpha-disk
in a binary system after the peak of an outburst, when matter in the disk is
totally ionized. The accretion rate in the disk decreases according to a power
law. We derive formulas for the accretion rate and effective temperature of the
disk. The model has three free input parameters: the mass of the central object
M, the turbulence parameter alpha, and the normalization parameter delta t.
Results of the modeling are compared with the observed X-ray and optical B and
V light curves. The resulting estimates for the turbulence parameter $\alpha$
are similar: 0.2-0.4 for A 0620-00 and 0.45-0.65 for GS 1124-683, suggesting a
similar nature for the viscosity in the accretion disks around the compact
objects in these sources. We also derive the distances to these systems as
functions of the masses of their compact objects.
