  Pre-big bang cosmology predicts tiny first-order dilaton and metric
perturbations at very large scales. Here we discuss the possibility that other
-- more copiously generated -- perturbations may act, at second order, as
scalar seeds of large-scale structure and CMB anisotropies. We study, in
particular, the cases of electromagnetic and axionic seeds. We compute the
stochastic fluctuations of their energy-momentum tensor and determine the
resulting contributions to the multipole expansion of the temperature
anisotropy. In the axion case it is possible to obtain a flat or slightly
tilted blue spectrum that fits present data consistently, both for massless and
for massive (but very light) axions.
