  We investigate the generation of curvature and isocurvature (dilaton, moduli
and axion) perturbations in a general class of axion-dilaton-moduli
models,including the pre-big bang scenario. Allowing for an arbitrary coupling
constant between the dilaton field and the axion field, we exploit the SL(2,R)
symmetry of the theory to obtain the spectral indices of the field
perturbations in a pre-big bang type scenario. 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)xSL(2,R)
symmetry, where a second axion field arises from the compactification of the
ten-dimensional theory to four dimensions.
