  The so-called curvaton mechanism --a way to convert isocurvature
perturbations into adiabatic ones-- is investigated both analytically and
numerically in a pre-big bang scenario where the role of the curvaton is played
by a sufficiently massive Kalb--Ramond axion of superstring theory. When
combined with observations of CMBR anisotropies at large and moderate angular
scales, the present analysis allows us to constrain quite considerably the
parameter space of the model: in particular, the initial displacement of the
axion from the minimum of its potential and the rate of evolution of the
compactification volume during pre-big bang inflation. The combination of
theoretical and experimental constraints favours a slightly blue spectrum of
scalar perturbations, and/or a value of the string scale in the vicinity of the
SUSY-GUT scale.
