  The S--duality transformations of the lowest order string effective theory
admit a space time interpretation for 4-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 pure
gravitational. In this fashion, S-duality induces an $SL(2,R)$ transformation
in the space of target space metrics which coincides with the action of the
Ehlers-Geroch group and interchanges 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,
however, that the self-duality of the metric is not generically preserved,
unless the corresponding Killing vector field has self-dual covariant
derivatives. Thus, the T-S-T transformations are not always compatible with the
world-sheet supersymmetry of $N=4$ superconformal string vacua. We also provide
an algebraic characterization of the corresponding obstruction and associate it
with a breakdown of space time supersymmetry under rotational T-duality
transformations.
