  A primordial stochastic background of very weakly coupled massless
(pseudo-)scalars can seed CMB anisotropy, when large-scale fluctuations of
their stress-tensor re-enter the horizon during the matter-dominated era. A
general relation between multipole coefficients of the CMB anisotropy and the
seed's energy spectrum is derived. Magnitude and tilt of the observed
anisotropies can be reproduced for the nearly scale-invariant axion spectra
that are predicted in a particularly symmetric class of string cosmology
backgrounds.
