Closing a window for massive photons
  Working with the assumption of non-zero photon mass and a trajectory that is described by the non geodesic world line of a spinning top we find, by deriving new astrophysical bounds, that this assumption is in contradiction with current experimental results. This yields the conclusion that such photons have to be exactly massless. 
Sergio A. Hojman and Benjamin Koch
hep-ph astro-ph.CO gr-qc