2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/6285The mixing of the photon with a hypothetical sterile paraphotonic state would have consequences on the cosmological propagation of photons. Observations of gamma-rays from active galactic nuclei in GLAST and MAGIC will open a new domain in the search for such a phenomenon.4 pages, LatexAstrophysicstext