some dispersion around the characteristics of this quasar. It is well known (see e.g. Francis et al. 1991) that the population of quasars displays a variety of power-law flux distributions and that the emission-line equivalent widths vary from one object to the other both in a systematic way (the Baldwin effect) and in a random way. On the basis of the power-law index dispersion reported, e.g. by Francis et al. (1991), we expect this effect to spread the colours of quasars by $\pm~0.3$ mag around our tracks. In addition, particular realizations of the distribution of high column density clouds along some line of sight could induce strong deviations from the mean behaviour.

Colour transformations
======================

Colour transformations between a physical and a standard filter set always are a source of photometric errors (see e.g. Sterken & Manfroid 1992 or Royer & Manfroid 1996 and references therein). In the particular case of the XMM-OM optical filters, the important discrepancies between the standard and non-standard $U$, $B$ and $V$ filters renders the choice of adequate transformations even more critical. In order to avoid additional errors introduced by subsequent colour computations, we established colour transformations for both single filters and colour indices.

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