  The general prediction that more than half of all CVs have evolved past the
period minimum is in strong disagreement with observational surveys, which show
that the relative number of these objects is just a few per cent. Here, we
investigate whether a large number of post-period minimum CVs could detach
because of the appearance of a strong white dwarf magnetic field potentially
generated by a rotation- and crystallization-driven dynamo. We used the MESA
code to calculate evolutionary tracks of CVs incorporating the spin evolution
and cooling as well as compressional heating of the white dwarf. If the
conditions for the dynamo were met, we assumed that the emerging magnetic field
of the white dwarf connects to that of the companion star and incorporated the
corresponding synchronization torque, which transfers spin angular momentum to
the orbit. We find that for CVs with donor masses exceeding 0.04 Msun, magnetic
fields are generated mostly if the white dwarfs start to crystallize before the
onset of mass transfer. It is possible that a few white dwarf magnetic fields
are generated in the period gap. For the remaining CVs, the conditions for the
dynamo to work are met beyond the period minimum, when the accretion rate
decreased significantly. Synchronization torques cause these systems to detach
for several Gyrs even if the magnetic field strength of the white dwarf is just
one MG. If the rotation- and crystallization-driven dynamo - which is currently
the only mechanism that can explain several observational facts related to
magnetism in CVs and their progenitors - or a similar temperature-dependent
mechanism is responsible for the generation of magnetic field in white dwarfs,
most CVs that have evolved beyond the period minimum must detach for several
Gyrs at some point. This reduces the predicted number of semi-detached period
bouncers by up to 60-80 per cent.
