  We report the first study of Fourier-frequency-dependent coherence and
phase/time lags at optical wavelengths of cataclysmic variables (MV Lyr and LU
Cam) displaying typical flickering variability in white light. Observations
were performed on the William Herschel Telescope using ULTRACAM. Lightcurves
for both systems have been obtained with the SDSS filters $u'$, $g'$ and $r'$
simultaneously with cadences between $\approx0.5-2$ seconds, and allow us to
probe temporal frequencies between ~10^{-3} Hz and ~1 Hz. We find high levels
of coherence between the u', g' and r' lightcurves up to at least ~10^{-2} Hz.
Furthermore we detect red/negative lags where the redder bands lag the bluer
ones at the lowest observed frequencies. For MV Lyr time lags up to ~3 seconds
are observed, whilst LU Cam displays larger time lags of ~10 seconds.
Mechanisms which seek to explain red/negative lags observed in X-ray binaries
and Active Galactic Nuclei involve reflection of photons generated close to the
compact object onto the surface layers of the accretion disk, where the lag
delay is simply the light-travel time from the emitting source to the
reflecting accretion disk area. Although this could be a viable explanation for
the lags observed in MV Lyr, the lags observed in LU Cam are too large to be
explained by reflection from the disk and/or the donor star. We suggest
reprocessing on the thermal timescale of boundary layer photons onto the
accretion disk as a possible mechanism to explain the lags observed in
accreting white dwarfs, or reverse (inside-out) shocks within the disk
travelling through cooler disk regions as they move outwards.
