the Kurucz and FRV ($U-B$) and ($U-V$) colour indices are slightly discrepant below $5000$ K, as are the ($U-u$) and the ($uvw1-u$) colour indices below $7000$ K.

As a conclusion, before the full inflight calibration on standard fields is performed and reduced, we recommend to use the relations given above to analyse the first data provided by the XMM-OM.

Reddening and temperature determination
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In order to discriminate between stars having different reddening and temperature characteristics, a combination of colour indices must be found such that temperature tracks (i.e. curves of constant reddening) do not cross each other in the related colour diagram. In the XMM-OM photometric system, we found only one fully suitable pair of such colour indices: ($u-uvw1$) and ($b-v$). The corresponding colour diagram is shown in Fig. 4. This figure is representative of the weakest reddenings, but the diagram keeps its properties up to $A_{\rm
V}~\sim~13$. Only a slight adaptation of the lowest temperature bound ($9000$ K $\rightarrow~11000$ K) is necessary. We did not plot stars below $9000$ K in Fig. 4 because the temperature tracks of MS stars begin to overlay at this temperature, so that nothing can be said for lower temperatures. Uncertainties on the 