  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 0.02 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 the a nonstationary accretion disk behavior,
connected with a high ellipticity of the orbital motion. The ephemeris used in
this paper -- JD$_{\rm opt-outb}$ = JD$_0$(2,444,944) $\pm$ n(111.0 $\pm$ 0.4)
days are derived from the orbital period of the system P$_{\rm orb} = 111.0 \pm
0.4$ days, determined by Priedhorsky & Terrell (1983), and from the optical
flare of December 5, 1981 (Giovannelli et al., 1985) (here after 811205-E; E
stands for the Event occurred at that date) that triggered the subsequent X-ray
outburst of December 13, 1981 (Nagase et al., 1982) (here after 811213-E). 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 the matter in the
accretion disk, after an 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 $\alpha=0.1-0.3$. The increase of the mass flux
is a sort of flush that reaches the external part of the accretion disk around
the neutron star, producing an enhancement in the optical luminosity. The
consequent X-ray flare happens when the matter reaches the hot central parts of
the accretion disk, and the neutron star surface.
