Phenomenological consequences of a direct fifth force coupling to photons

The possible consequences of a mixing between the photon field and the vector component of a fifth-force field are considered, using an effective phenomenological model based on the coupling term $F_{\mu \nu }{G}^{\mu \nu }$, which may be motivated in the context of multidimensional and supersymmetric unification schemes. Various constraints on the strength of this coupling, following from observational data, are discussed.