2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/5722The nature of the ultraluminous X-ray sources (ULX) is still to be understood. Several hypotheses have been suggested to explain the high X-ray luminosity: stellar mass black holes with emission collimated by relativistic effects or anisotropies in the accretion disk, intermediate mass black holes, young supernovae remnants, background active galactic nuclei. To discriminate among the above hypotheses, the search for relativistic effects can be a valuable "pathfinder". Here we propose a list of ULX candidates suitable to show relativistic beaming5 pages, no figure. For the Proceedings of the 15th SIGRAV Conference on General Relativity and Gravitation. Corrected for a misspelled referenceAstrophysicstext