  The optical behaviour of the Be star in the high mass X-ray transient
A0535+26/HDE245770 shows that at the periastron typically there is an
enhancement in the luminosity of order 0.05 to few tenths mag, and the X- ray
outburst happens about 8 days after the periastron. We construct a quantitative
model of this event, basing on a nonstationary accretion disk behavior,
connected with a high ellipticity of the orbital motion. We explain the
observed time delay between the peaks of the optical and X-ray outbursts in
this system by the time of radial motion of a matter in the accretion disk,
after increase of the mass flux in the vicinity of a periastral point in the
binary. This time is determined by the turbulent viscosity, with the parameter
$ \alpha=0.1-0.3$, estimated from the comparison of the model with
observational data
