Adiabatic CMB perturbations in pre-big bang string cosmology

We consider the pre-big bang scenario with a massive axion field which starts to dominate energy density when oscillating in an instanton-induced potential and subsequently reheats the universe as it decays into photons, thus creating adiabatic CMB perturbations. We find that the fluctuations in the axion field can give rise to a nearly flat spectrum of adiabatic perturbations with a spectral tilt $\Delta n$ in the range $-0.1 \lesssim \Delta n \lesssim 0.3$.