  We study the center symmetry of SU(N) gauge theories with fermions in the
two-index representations, by computing the effective potential of the Polyakov
loop in the large-mass expansion on the lattice. In the large-N limit and at
non-zero temperature, we find that the center symmetry is Z_N for fermions in
the adjoint representation and just Z_2 for fermions in the (anti)symmetric
representation. We discuss the fact that our results do not contradict the
orientifold planar equivalence, which relates a common sector defined by the
bosonic gauge-invariant C-even states of theories with fermions in different
two-index representations. Our results complement the work of Armoni et al.
(2007), who showed how at zero temperature a Z_N center symmetry is dynamically
recovered also for fermions in the (anti)symmetric representation, by
considering the theories at finite temperature.
