Constraints on the photon charge based on observations of extragalactic   sources
  Using modern high-resolution observations of extragalactic compact radio sources we obtain an estimate of the upper bound on a photon electric charge at the level $e_{\gamma} \lesssim 3 \cdot 10^{-33}$ of elementary charge (assuming the photon charge to be energy independent). This is three orders of magnitude better than the limit obtained with radio pulsar timing. Also we set a limit on a photon charge in the gamma-ray band (energies about 0.1 MeV). In future the estimate made for extragalactic sources can be significantly improved. 
V.V. Kobychev (1), S.B. Popov (2,3) ((1)Institute for Nuclear Research   of Ukrainian NAS; (2)University of Padova, (3)Stern...