  We study, for the first time, the phase structure of the Gross--Neveu model
with a combination of a (constant) gravitational and a magnetic field. This has
been made possible by our finding of an exact solution to the problem, namely
the effective potential for the composite fermions. Then, from the
corresponding implicit equation the phase diagram for the dynamical fermion
mass is calculated numerically for some values of the magnetic field. %(what
can be done with arbitrary precision). For a small magnetic field the phase
diagram hints to the possibility of a second order phase transition at some
critical curvature. With growing magnetic field only the phase with broken
chiral symmetry survives, because the magnetic field prevents the decay of the
chiral condensate. This result is bound to have important consequences in early
universe cosmology.
