Using light rings and photon-spheres to tell apart Kerr from non-Kerr spacetimes.

The recent observations of the EHT have opened a new frontier in testing GR and its possible modifications, using the photons that come form the vicinity of supermassive BHs, such as the one at the centre of M87 and that of our Milky Way, Sgr A*. Here we present some situations under which the properties of the photon orbits of a non-Kerr object may be quite different from those of a Kerr BH or in some sense of a Kerr-like object, essentially providing a smoking gun signal for some types of deviations from the Kerr solution.