If photons have a small electric charge ${\mathit{Q}}_{\gamma }$ their path in the galactic magnetic field will 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 ${\mathit{Q}}_{\gamma }$/e≲${10}^{\mathrm{-}29}$.

• Received 2 September 1994

DOI:https://doi.org/10.1103/PhysRevD.50.7729