their physical characteristics are summarized in Table 1. Throughout the rest of this paper, small characters refer to the XMM-OM filters and capital letters refer to the standard ones (i.e. the $U$, $B$ and $V$ taken from Bessel 1990).

       filter       $\lambda_{o}$   $\lambda_{\rm max}$      FWHM
  ---------------- --------------- --------------------- -------------
   [*[uvw2]{}*]{}       2070               2000           $\sim\,500$
   [*[uvm2]{}*]{}       2298               2210               439
   [*[uvw1]{}*]{}       2905               2680               620
    [*[u]{}*]{}         3472               3270               810
    [*[b]{}*]{}         4334               3980               976
    [*[v]{}*]{}         5407               5230               684

  : Main characteristics of the XMM-OM filters. The wavelengths are expressed in Å. $\lambda_0$ is the effective wavelength and $\lambda_{\rm max}$ is the position of the maximal transmission.

The XMM-OM filter set consists of 3 UV and 3 optical filters, further denoted by $uvw2$, $uvm2$, $uvw1$ and $u$, $b$, $v$ respectively, in order of increasing central wavelength. The $u$, $b$ and $v$ filters are intended to match the Johnson $U$, $B$ and $V$ (Bessel 1990) filters respectively, but they possess more or less rectangularly shaped passbands, which greatly affects the quality of the match and of the subsequent colour transformations between the two systems. This is especially true for what concerns the (mis)match between the Johnson $U$ and XMM-OM $u$ filters and thus the $(U-u)$ and $(U-B)$ related 