Gravity Degrees of Freedom on a Null Surface
  A canonical analysis for general relativity is performed on a null surface without fixing the diffeomorphism gauge, and the canonical pairs of configuration and momentum variables are derived. Next to the well-known spin-2 pair, also spin-1 and spin-0 pairs are identified. The boundary action for a null boundary segment of spacetime is obtained, including terms on codimension two corners. 
Florian Hopfm\"uller, Laurent Freidel
gr-qc hep-th