  We showed in a previous paper that a wide class of nonmetric theories of
gravity encompassed by the $\chi g$ formalism predict that the speed of light
rays depends on the their polarization direction relative to directions singled
out by the gravitational field. This gravity-induced birefringence of space is
a violation of the Equivalence Principle and is due to the nonmetric coupling
between gravity and electromagnetism. In this paper we analyze the propagation
of light in the gravitational field of a rotating black hole when nonmetric
couplings to curvature are included and compute the time delay between rays
with orthogonal polarizations. We obtain an upper bound on the strength of QED
like and CP violating curvature couplings using time delay data for pulsar PSR
1937+21. By comparison the corresponding coupling strength for QED coupling is
37 orders of magnitude less.
