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 1 .