  In this paper we study cosmological signatures of modified gravity theories
that can be written as a coupling between a extra scalar field and the
electromagnetic part of the usual Lagrangian for the matter fields. In these
frameworks all the electromagnetic sector of the theory is affected and
variations of fundamental constants, of the cosmic distance duality relation
and of the evolution law of the cosmic microwave background radiation (CMB) are
expected and are related each other. In order to search these variations we
perform jointly analyses with angular diameter distances of galaxy clusters,
luminosity distances of type Ia supernovae and $T_{CMB}(z)$ measurements. We
obtain tight constraints with no indication of violation of the standard
framework.
