A cosmological bound on $e^+ e^-$ mass difference
  We demonstrate that CPT-violation due to $e^+ e^-$ mass difference generates a non-zero photon mass. As a result the cosmological bounds on the photon mass lead to the bounds on $e^+ e^-$ mass difference which are at least by 10 orders of magnitude stronger than the direct experimental bound. 
A.D. Dolgov, V.A. Novikov
hep-ph astro-ph.CO hep-th