Sergei B. Popov is qualified to endorse.

Constraints on the photon charge based on observations of extragalactic sources

Vladislav Kobychev:	Is registered as an author of this paper. Not currently an endorser. (why?)
Sergei B. Popov:	Is registered as an author of this paper. Can endorse for astro-ph, astro-ph.CO, astro-ph.EP, astro-ph.GA, astro-ph.HE, astro-ph.IM, astro-ph.SR. (why?)