Lorentzian metrics from characteristic surfaces

The following issue is raised and discussed; when do families of foliationsby hypersurfaces on a given four dimensional manifold become the null surfacesof some unknown, but to be determined, metric $g_{ab}(x)$? It follows fromthese results that one can use these surfaces as fundamental variables for GR.Comment: 14 pages, tarred, compressed and uuencode RTF fil