Cerenkov radiation by charged particles in an external gravitational field

Charged particles in the geodesic trajectory of an external gravitational field do not emit electromagnetic radiation. This is expected from the application of the equivalence principle. We show here that charged particles propagating in an external gravitational field with non-zero components of the Ricci tensor can emit radiation by the \v{C}erenkov process. The external gravitational field acts like an effective refractive index for light. Since the Ricci tensor cannot be eliminated by a change of coordinates, there is no violation of the equivalence principle in this process.