  We derive power density spectra in the optical and X-ray band in the
timescale range from several years down to $\sim $ a day. We suggest that the
optical power density spectrum consists of two separate components: long
timescale variations and short timescale variations, with the dividing
timescale around 100 days. The shape of the short timescale component is
similar to X-ray power density spectrum which is consistent with the
interpretation of short timescale optical variations being caused by X-ray
reprocessing. We show that the observed optical long timescale variability is
consistent with thermal pulsations of the accretion disc.
