  We consider a charged particle, spin 1/2, with relativistic kinetic energy
and minimally coupled to the quantized Maxwell field. Since the total momentum
is conserved, the Hamiltonian admits a fiber decomposition as $H(P)$, $P\in
\BbbR^3$. We study the spectrum of $H(P)$.
  In particular we prove that, for non-zero photon mass, the ground state is
exactly two-fold degenerate and separated by a gap, uniformly in $P$, from the
rest of the spectrum.
