  Although the internal structure of white dwarfs is considered to be generally
well understood, the source and entity of viscosity is still very uncertain. We
propose here to study white dwarf viscous properties using short period (< 1
hr), detached white dwarf binaries, such as the newly discovered ~12.8 min
system. These binaries are wide enough that mass transfer has not yet started
but close enough that the least massive component is subject to a measurable
tidal deformation. The associated tidal torque transfers orbital energy, which
is partially converted into heat by the action of viscosity within the deformed
star. As a consequence, its outer non-degenerate layers expand, and the star
puffs up. We self-consistently calculate the fractional change in radius, and
the degree of asynchronism (ratio of stellar to orbital spin) as a function of
the viscous time. Specializing our calculations to J0651, we find that the
discrepancy between the measured radius of the secondary star and He white
dwarf model predictions can be interpreted as tidal inflation if the viscous
timescale is either ~2 10^5 yr or ~10^4 yr. Such values point to a
non-microscopic viscosity, possibly given by tidally induced turbulence, or by
magnetic field stresses with a magnetic field strength of 10-100 Gauss.
Fortunately, these two timescales produce very different degree of
asynchronism, with the shortest one, bringing the system much closer to
synchronisation. A measurement of the stellar spin can thus univocally
determined the mean viscosity. Extrapolating the secondary's radial expansion,
we predict that the star will fill is Roche lobe at a separation which is
1.2-1.3 smaller than the current one. Applying this method to a future sample
of systems can allow us to learn whether viscosity changes with mass and/or
nuclear composition.
