  Low energy string theory predicts the existence of an axion field which can
lead to cosmic birefringence. We solve the electromagnetic wave equations in
the presence of such an axion and a dilaton field in order to determine their
effect on the polarization of light. We find that the presence of dilaton field
leads to a nontrivial modification of the final result. We comment on the
possibility of discovering such an effect by observations of radio wave
polarizations from distant radio galaxies and quasars. We have also determined
the limits on the string theory parameters that are imposed by the current
polarization data from distant quasars and radio galaxies.
