  The SL(2,R) invaraint ten dimensional type IIB superstring effective action
is compactified on a torus to D spacetime dimensions. The transformation
properties of scalar, vector and tensor fields, appearing after the dimensional
reduction, are obtained in order to maintain the SL(2,R)} invariance of the
reduced effective action. The symmetry of the action enables one to generate
new string vacua from known configurations. As illustrative examples, new black
hole solutions are obtained in five and four dimensions from a given set of
solutions of the equations of motion.
