  We analyse the boundary structure of General Relativity in the coframe
formalism in the case of a lightlike boundary, i.e., when the restriction of
the induced Lorentzian metric to the boundary is degenerate. We describe the
associated reduced phase space in terms of constraints on the symplectic space
of boundary fields. We explicitly compute the Poisson brackets of the
constraints and identify the first- and second-class ones. In particular, in
the 3+1 dimensional case, we show that the reduced phase space has two local
degrees of freedom, instead of the usual four in the non-degenerate case.
