  To shed light on the origin of magnetic symbiotic stars, we investigated the
system FN Sgr in detail. We searched for a reasonable formation pathway to
explain its stellar and binary parameters including the magnetic field of the
accreting white dwarf. We used the MESA code to carry out pre-CE and post-CE
binary evolution and determined the outcome of CE evolution assuming the energy
formalism. For the origin and evolution of the white dwarf magnetic field, we
adopted the crystallization scenario. We found that FN Sgr can be explained as
follows. First, a non-magnetic white dwarf is formed through CE evolution.
Later, during post-CE evolution, the white dwarf starts to crystallize and a
weak magnetic field is generated. After a few hundred Myr, the magnetic field
penetrates the white dwarf surface and becomes detectable. Meanwhile, its
companion evolves and becomes an evolved red giant. Subsequently, the white
dwarf accretes part of the angular momentum from the red giant stellar winds.
As a result, the white dwarf spin period decreases and its magnetic field
reaches super-equipartition, getting amplified due to a rotation- and
crystallization-driven dynamo. The binary then evolves into a symbiotic star,
with a magnetic white dwarf accreting from an evolved red giant through
atmospheric Roche-lobe overflow. We conclude that the rotation- and
crystallization-driven dynamo scenario, or any age-dependent scenario, can
explain the origin of magnetic symbiotic stars reasonably well. This adds
another piece to the pile of evidence supporting this scenario. If our
formation channel is correct, our findings suggest that white dwarfs in most
symbiotic stars formed through CE evolution might be magnetic, provided that
the red giant has spent >3 Gyr as a main-sequence star.
