The effect of the massless gravitational scalar field assumed to couple directly to the Maxwell field on the solar-system experiments is estimated. We start with discussing the theoretical significance of this coupling. Rather disappointingly, however, we find that the scalar-field parameters never affect the observation in the limit of geometric optics, indicating a marked difference from the well-known contribution through the spacetime metric.

• Received 9 September 2006

DOI:https://doi.org/10.1103/PhysRevD.75.064028