  We present the results of a detailed study of how isocurvature axion
fluctuations are converted into adiabatic metric perturbations through axion
decay, and discuss the constraints on the parameters of pre-big bang cosmology
needed for consistency with present CMB-anisotropy data. The large-scale
normalization of temperature fluctuations has a non-trivial dependence both on
the mass and on the initial value of the axion. In the simplest, minimal models
of pre-big bang inflation, consistency with the COBE normalization requires a
slightly tilted (blue) spectrum, while a strictly scale-invariant spectrum
requires mild modifications of the minimal backgrounds at large curvature
and/or string coupling.
