I report recent studies on the evolution of perturbations in the context of the ``pre-big-bang" scenario typical of string cosmology, with emphasis on the formation of a stochastic background of relic photons and gravitons, and its possible direct/indirect observable consequences. I also discuss the possible generation of a thermal microwave background by using, as example, a simple gravi-axio-dilaton model whose classical evolution connects smoothly inflationary expansion to decelerated contraction. By including the quantum back-reaction of the produced radiation the model eventually approaches the standard radiation-dominated (constant dilaton) regime.