  Accreting white dwarfs are known to show signatures of wind-type outflows in
the ultraviolet. At optical wavelengths, however, wind detections have only
been reported for a few sources. We present GTC-10.4m optical spectroscopy of
four accreting white dwarfs (BZ Cam, V751 Cyg, MV Lyr, and V425 Cas) observed
during luminous epochs, when their optical emission is expected to be dominated
by the accretion disc. We focused the analysis on four emission lines:
H$\alpha$ and He I $\lambda$5876, $\lambda$6678, $\lambda$7065. Line profiles
are complex and variable on short (minutes) and long (days to weeks) time
scales, with transient absorption and emission components. Among them, we
detect strong blue-shifted absorptions at $\gtrsim 1000$ km s$^{-1}$. These
high-velocity components, present only in the blue wing of the emission lines,
are observed in all four sources and could be associated with accretion disc
winds. For MV Lyr and V425 Cas, these would represent the first detection of
optical outflows in these objects, while in the case of BZ Cam and V751 Cyg,
the presence of outflows has been previously reported. This study suggests
that, in addition to ultraviolet winds, optical outflows might be also common
in accreting white dwarfs. We discuss the observational properties of these
winds and their possible similarity to those detected in accreting black holes
and neutrons stars.
