  To understand the physical processes governing accretion discs we can study
active galactic nuclei (AGN), X-ray binary systems (XRBs) and cataclysmic
variables (CVs). It has been shown that XRBs and CVs show similar observational
properties such as recurrent outbursts and aperiodic variability. The latter
has been extensively studied for XRBs, but only recently have direct
phenomenological analogies been found between XRBs and CVs, including the
discovery of the rms--flux relation and the optical detection of
Fourier-dependent time-lags. We present a Fourier analysis of the well-known CV
SS Cyg in quiescence based on data collected at the 4.2--m William Herschel
Telescope using ULTRACAM. Light curves in SDSS filters $u'$, $g'$ and $r'$ were
taken simultaneously with sub-second cadence. The high cadence and sensitivity
of the camera allow us to study the broad-band noise component of the source in
the time range $\sim 10000 - 0.5$ s ($10^{-4}-2$ Hz). Soft/negative lags with
an amplitude ~ 5 s at a time-scale of ~ 250 s were observed, indicating that
the emission in the redder bands lags the emission in the bluer bands. This
effect could be explained by thermal reprocessing of hard photons in the
innermost region of the accretion disc, assuming a high viscosity parameter
$\alpha > 0.3$, and high irradiation of the disc. Alternatively, it could be
associated with the recombination time-scale on the upper layer of the
accretions disc.
