  Conformal invariance for bosonic strings in time-dependent backgrounds of
graviton, dilaton and Kalb-Ramond field is obtained by imposing Weyl-beta
functions to be homogeneous in time, to all orders in $\alpha^{'}$. This
construction is possible in any target space dimension, as a result of the
non-trivial background configurations. The electromagnetic effects of the
antisymmetric tensor field, when coupled to an Abelian (electromagnetic) gauge
field, are discussed in the framework of a four-dimensional Minkowski Universe,
for concreteness. Non-trivial optical activity is demonstrated, which
constitutes a way of detecting string-inspired axion/dilaton effects in such
models.
