Variation of the speed of light due to non-minimal coupling between electromagnetism and gravity

We consider an Einstein-Maxwell action modified by the addition of three terms coupling the electromagnetic strength to the curvature tensor. The corresponding generalized Maxwell equations imply a variation of the speed of light in a vacuum. We determine this variation in Friedmann-Robertson-Walker spacetimes. We show that light propagates at a speed greater than $c$ when a simple condition is satisfied.