  The origin of strong magnetic fields in white dwarfs has been a puzzle for
decades. Recently, a dynamo mechanism operating in rapidly rotating and
crystallizing white dwarfs has been suggested to explain the occurrence rates
of strong magnetic fields in white dwarfs with close low-mass main sequence
star companions. Here we investigate whether the same mechanism may produce
strong magnetic fields in close double white dwarfs. The only known strongly
magnetic white dwarf that is part of a close double white dwarf system, the
magnetic component of NLTT 12758, is rapidly rotating and likely crystallizing
and therefore the proposed dynamo mechanism represents an excellent scenario
for the origin of its magnetic field. Presenting a revised formation scenario
for NLTT 12758, we find a natural explanation for the rapid rotation of the
magnetic component. We furthermore show that it is not surprising that strong
magnetic fields have not been detected in all other known double white dwarfs.
We therefore conclude that the incidence of magnetic fields in close double
white dwarfs supports the idea that a rotation and crystallization driven
dynamo plays a major role in the generation of strong magnetic fields in white
dwarfs.
