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

Astrophysics;General Relativity and Quantum Cosmology