of rectangularly shaped filters), or even not always possible. Such colour transformations will nevertheless remain necessary for those who will have to compare XMM-OM observations with ground-based ones. In this paper, we provide the reader with theoretical estimations of these colour transformations.

The XMM-OM filter set comprises three non-standard filters, exploring a wavelength domain (UV) where no extensive observations have been performed to date. Combined with the poor match between the XMM-OM optical filters and the classical ground-based standard photometric systems, this fact enhances the importance of testing our ability to determine some physical properties of the observed stars directly in the natural XMM-OM colour system. In the present paper, we will show that it is possible to estimate both the temperature and the amount of interstellar absorption (or “reddening”) for the hot stars that will be observed with XMM-OM.

The natural XMM-OM colour system can also be used for other investigations. For example, we have explored the possibilities to discriminate quasars from stars in multidimensional colour spaces based on the XMM-OM photometry. Nevertheless, the goal of this paper is not so much to provide the reader with exact analytical relations for these matters as to give qualitative results that 