  Starting from an exact (in the Regge slope alpha') functional method for a
bosonic stringy sigma-model, we investigate four-dimensional cosmological
string solutions in graviton, dilaton and antisymmetric tensor backgrounds,
compatible with world-sheet conformal invariance, and valid beyond perturbative
expansions in powers of alpha'. The antisymmetric tensor field, playing the
role of an axion in the four-dimensional target space time, leads to spatial
anisotropies of the emergent Robertson-Walker expanding Universe, and, upon
coupling the system to the electromagnetic field, it results in non-trivial
optical activity. Some estimates of the corresponding effects are made and
their relevance to current cosmology is briefly discussed.
