DISCONTINUITIES AND COLLISION OF GRAVITATIONAL-WAVES IN STRING THEORY

We examine here discontinuities in the metric, the antisymmetric tenso r and the dilaton field which are allowed by conformal invariance. We find that the surfaces of discontinuity must necessarily be null and b oth shock and impulsive waves are allowed. We employ our results for t he case of colliding plane gravitational waves and we discuss the SL(2 ,R) x SU(2)/R x R WZW model in the present perspective. In particular, the singularities encountered in this model may be viewed as the resu lt of the mutual focusing of the colliding waves.