will tell him how to observe and treat the data in order to get the best outcome, and what kind of results are to be expected.

In Sect. 2, we present the tools we used for the synthetic photometry. In Sect. 3, we discuss the colour transformations between the XMM-OM optical filters and the Johnson $U$, $B$, $V$ system. We treat the temperature and the interstellar absorption determination in Sects. 4 and 5 whereas the selection of quasar candidates is addressed in Sect. 6. Sect. 7 outlines the main conclusions of our study.

The basic information and hypotheses
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The filters
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All simulations presented hereafter were established on the basis of synthetic photometry. The filter passbands that we used for the XMM-OM photometric system are extracted from the XMM Users’ Handbook (Dahlem & Schartel 1999). Their transmission curves result from the product of the transmission curves and/or sensitivity curves of all physical devices in the XMM-OM light path. Namely, we have the filters themselves, three aluminized mirrors, one window at the entry of the detector and the detector response curve. All these curves originate from the same reference as above. The resulting passbands are plotted in Fig. 1a and 