SPACE-TIME INTERPRETATION OF S-DUALITY AND SUPERSYMMETRY VIOLATIONS OF T-DUALITY

The S-duality transformations of the lowest order string effective the ory admit a space time interpretation for ii-dim backgrounds with one Killing symmetry. Starting from pure gravity and performing a sequence of intertwined T-S-T duality transformations we obtain new solutions which are always purely gravitational. In this fashion, S-duality indu ces an SL(2,R) transformation in the space of target space metrics, wh ich coincides with the action of the Ehlers-Geroch group and interchan ges the electric with the magnetic aspects of gravity. Specializing to gravitational instanton backgrounds, we show that ALE instantons are mapped to (multi) Taub-NUT backgrounds and vice versa. We find, howeve r, that the self-duality of the metric is not generically preserved, u nless the corresponding Killing vector field has self-dual covariant d erivatives. Thus, the T-S-T transformations are not always compatible with the world-sheet supersymmetry of N = 4 superconformal string vacu a. We also provide an algebraic characterization of the corresponding obstruction and associate it with a breakdown of space time supersymme try under rotational T-duality transformations.