Constraints on non-minimally coupled curved space electrodynamics from astrophysical observations

We study interactions of electromagnetic fields with the curvature tensor of the form (lambda)R(mu)(nu)(alpha)(beta)F(mu)(nu)F(alpha)(beta). Such coupling terms, though invariant under general coordinate transformation and CPT, 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.