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abstract: '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 several aspects. First, we discuss XMM-OM’s photometric system and its colour transformations towards the standard $U\!BV$ system. Second, we establish a set 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-OM filter set.'
author:
- 'P. Royer [^1]'
- 'J. Manfroid [^2]'
- 'E. Gosset [^3]'
- 'J.-M. Vreux'
date: 'Received date; accepted date'
title: 'Simulations of Observations with the Optical Monitor of the X-Ray Multi-Mirror Satellite'
---

Introduction
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Thanks to its unprecedented large X-ray collecting power, the XMM satellite (for X-ray Multi-Mirror) is expected to discover a wealth of new X-ray sources. In order to allow a quick and reliable identification of the sources, as well as to enable multiwavelength monitoring of their variability, the satellite is equipped with an optical complement known as the optical monitor, XMM-OM. This particular element is co-aligned with the three X-ray telescopes and consists of a 30 cm Ritchey-Chrétien telescope coupled with a photon-counting 