  We present the results of a study of the low-mass X-ray binary 4U 1636-536.
We have performed temporal analysis of all available RXTE/ASM, Swift/BAT and
MAXI data. We have confirmed the previously discovered quasi-periodicity of ~45
d present during ~2004, however we found it continued to 2006. At other epochs,
the quasi-periodicity is only transient, and the quasi-period, if present,
drifts. We have then applied a time-dependent accretion disc model to the
interval with the significant X-ray quasi-periodicity. For our best model, the
period and the amplitude of the theoretical light curve agree well with that
observed. The modelled quasi-periodicity is due to the hydrogen
thermal-ionization instability occurring in outer regions of the accretion
disc. The model parameters are the average mass accretion rate (estimated from
the light curves), and the accretion disc viscosity parameters, for the hot and
cold phases. Our best model gives relatively low values of viscosity parameter
for cold phase 0.01 and for hot phase 0.03.
