S-DUALITY-INVARIANT PERTURBATIONS IN STRING COSMOLOGY

We investigate the generation of curvature and isocurvature (dilaton, moduli and axion) perturbations in a general class of axion-dilaton-mo duli models, including the pre-big bang scenario. Allowing for an arbi trary coupling constant lambda between the dilaton field and the axion field, we exploit the SL(2,R) symmetry of the theory to obtain the sp ectral indices of the field perturbations in a pre-big bang type scena rio. Axion field fluctuations about a homogeneous background field can yield a scale-invariant (Harrison-Zel'dovich) spectrum. As an example we present a string-motivated case with SL(2,R) x SL(2,R) symmetry, w here a second axion field arises from the compactification of the ten- dimensional theory to four dimensions. (C) 1997 Elsevier Science B.V.