  We use the strong coupling and hopping parameter expansions to calculate the
pion and rho meson masses for lattice Yang-Mills gauge theories with fermions
in irreducible two-index representations, namely the adjoint, symmetric and
antisymmetric. The results are found to be consistent with orientifold planar
equivalence, and leading order 1/N corrections are calculated in the lattice
phase. An estimate of the critical bare mass, for which the pion is massless,
is obtained as a function of the bare coupling. A comparison to data from the
two-flavour SU(2) theory with adjoint fermions gives evidence for a bulk phase
transition at beta~2, separating a pure lattice phase from a phase smoothly
connected to the continuum.
