Simulations of observations with the Optical Monitor of the X-ray Multi-Mirror Satellite

This paper addresses the question of the observations to be performed with the Optical Monitor (OM) of the X-ray Multi-Mirror Satellite (XMM) under se veral aspects, First, we discuss XMM-OM's photometric system and its colour transformations towards the standard UBV system. Second, we establish a se t of procedures to determine the temperature and the amount of interstellar absorption affecting the observed stars, Last, we address the possibility of isolating quasars in multidimensional colour diagrams based on the XMM-O M filter set.