Pulsar Bound on the Photon Electric Charge Reexamined
  If photons had a small electric charge $Q_\gamma$ their path in the galactic magnetic field would be curved, leading to a time delay between photons of different frequency from a distant source. Cocconi's previous application of this argument led to a limit which is too restrictive by a factor of about 200; the corrected bound is $Q_\gamma/e\lapprox10^{-29}$. 
Georg G. Raffelt