  We study interactions of electro-magnetic fields with the curvature tensor of
the form $\lambda R_{\mu \nu \alpha \beta}F^{\mu \nu}F^{\alpha \beta}$. Such
coupling terms though are invariant under general coordinate transformation and
CPT, however violate the Einstein equivalence principle. These couplings do not
cause any energy dependent dispersion of photons but they exhibit
birefringence. We put constraints on the coupling constant $\lambda$ using
results from solar system radar ranging experiments and millisecond-pulsar
observations. We find that the most stringent constraint comes from pulsar
observations and is given by $ \lambda < 10^{11} cm^2 $ obtained from the
timing of binary pulsar PSR B1534+12.
