[en] If dark energy couples to the fields of the standard model we can hope to detect or constrain it through non-gravitational effects. If the dark energy field couples to photons it behaves as an Axion-Like-Particle (ALP). ALPs mix with photons in the presence of magnetic fields and hence affect astronomical observations. We show that empirically established luminosity relations can be used as a new test for ALPs and that when applied to observations of active galactic nuclei this is highly suggestive of the existence of a very light ALP. (orig.)