In curvaton models the convertion of isocurvature perturbations into adiabatic ones takes place through the decay of an auxiliary massive scalar field. In the Pre-Big Bang scenario, the role of the curvaton can be naturally played by the Kalb-Ramond axion present in the low energy effective action of superstring theory. A complete analysis of the coupled equations of axion and metric perturbations shows that a scale-invariant adiabatic spectrum for scalar perturbations can be obtained in this context. Recent observations of CMB anisotropies are used to constrain the parameter space of the model.
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