SYMMETRIES OF IMPULSIVE GRAVITATIONAL-WAVES

We give a classification of pp-waves, where the curvature is concentra ted on a null plane, in terms of their Killing symmetries. Our approac h is based on the analysis of the group of normal-form-preserving diff eomorphisms. In order to obtain a complete classification it is necess ary to extend the symmetry-concept to allow for discontinuous Killing fields.