  A magnetic field, coherent over the horizon size at the decoupling and strong
enough to rotate the polarization plane of the CMBR, can be generated from the
electromagnetic vacuum fluctuations amplified by the space-time evolution of
the dilaton coupling. The possible relevance of this result for superstring
inspired cosmological models is discussed. Particular attention will be paid to
the connection between Faraday rotation signals and stochastic gravity-wave
backgrounds.
