  White dwarfs (WDs) can increase their mass by accretion from companion stars,
provided the mass-accretion rate is high enough to avoid nova eruptions. The
accretion regimes that allow growth of the WDs are usually calculated assuming
constant mass-transfer rates. However, it is possible that these systems are
influenced by effects that cause the rate to fluctuate on various timescales.
We investigate how long-term mass-transfer variability affects accreting WDs
systems. We show that, if such variability is present, it expands the parameter
space of binaries where the WD can effectively increase its mass. Furthermore,
we find that the supernova type Ia (SNIa) rate is enhanced by a factor 2-2.5 to
a rate that is comparable with the lower limit of the observed rates. The
changes in the delay-time distribution allow for more SNIae in stellar
populations with ages of a few Gyr. Thus, mass-transfer variability gives rise
to a new formation channel of SNIa events that can significantly contribute to
the SNIa rate. Mass-transfer variability is also likely to affect other binary
populations through enhanced WD growth. For example, it may explain why WDs in
cataclysmic variables are observed to be more massive than single WDs, on
average.
