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Although the internal structure of white dwarfs is considered to be generallywell understood, the source and entity of viscosity is still very uncertain. Wepropose here to study white dwarf viscous properties using short period (< 1hr), detached white dwarf binaries, such as the newly discovered ~12.8 minsystem. These binaries are wide enough that mass transfer has not yet startedbut close enough that the least massive component is subject to a measurabletidal deformation. The associated tidal torque transfers orbital energy, whichis partially converted into heat by the action of viscosity within the deformedstar. As a consequence, its outer non-degenerate layers expand, and the starpuffs up. We self-consistently calculate the fractional change in radius, andthe degree of asynchronism (ratio of stellar to orbital spin) as a function ofthe viscous time. Specializing our calculations to J0651, we find that thediscrepancy between the measured radius of the secondary star and He whitedwarf model predictions can be interpreted as tidal inflation if the viscoustimescale is either ~2 10^5 yr or ~10^4 yr. Such values point to anon-microscopic viscosity, possibly given by tidally induced turbulence, or bymagnetic field stresses with a magnetic field strength of 10-100 Gauss.Fortunately, these two timescales produce very different degree ofasynchronism, with the shortest one, bringing the system much closer tosynchronisation. A measurement of the stellar spin can thus univocallydetermined the mean viscosity. Extrapolating the secondary's radial expansion,we predict that the star will fill is Roche lobe at a separation which is1.2-1.3 smaller than the current one. Applying this method to a future sampleof systems can allow us to learn whether viscosity changes with mass and/ornuclear composition.

Constraining white dwarf viscosity through tidal heating in detached binary systems