  We analyse the evolution of close binary systems containing a neutron star
that lead to the formation of redback pulsars. Recently there has been some
debate on the origin of such systems and the formation mechanism of redbacks
may still be considered as an open problem. We show that the operation of a
strong evaporation mechanism, starting from the moment when the donor star
becomes fully convective (or alternatively since the formation of the neutron
star by accretion induced collapse), produces systems with donor masses and
orbital periods in the range corresponding to redbacks with donors appreciably
smaller than their Roche lobes, i.e., they have low filling factors (lower than
$0.75$). Models of redback pulsars can be constructed assuming the occurrence
of irradiation feedback. They have been shown to undergo cyclic mass transfer
during the epoch at which they attain donor masses and orbital periods
corresponding to redbacks, and stay in quasi-Roche lobe overflow conditions
with {\it high} filling factors. We show that, if irradiation feedback occurs
and radio ejection inhibits further accretion onto the neutron star after the
first mass transfer cycle, the redback systems feature {\it high} filling
factors. We suggest that the filling factor should be considered as a useful
tool for discriminating among those redback formation mechanisms. We compare
theoretical results with available observations, and conclude that observations
tend to favour models with high filling factors.
