  The exciton-polariton modes of a quantum dot lattice embedded in a planar
optical cavity are theoretically investigated. Umklapp terms, in which an
exciton interacts with many cavity modes differing by reciprocal lattice
vectors, appear in the Hamiltonian due to the periodicity of the dot lattice.
We focus on Bragg polariton modes obtained by tuning the exciton and the cavity
modes into resonance at high symmetry points of the Brillouin Zone. Depending
on the microcavity design these polaritons modes at finite in-plane momentum
can be guided and can have long lifetimes. Moreover, their effective mass can
be extremely small, of the order of $10^{-8} m_0$ ($m_0$ is the bare electron
mass), and they constitute the lightest exciton-like quasi-particles in solids.
