reddening determination for MS stars below $\sim12-13000$ K, where temperature tracks for MS stars and giants begin to significantly differ from each other.

Other discriminant colour indices than those of Fig. 4 exist in the system, but their combination with other ones only allows to determine the temperature and the reddening of the observed stars on much more restricted ranges of reddening and/or temperature. One can nevertheless design another, independent, reddening determination technique. In Fig. 4, reddening and temperature influence both colour indices plotted on the axes. As we will see now, one can nearly decouple these parameters and obtain better precision on the reddening. To do so, we need to define a colour index that is independent of reddening and another that is proportional to it. The former, that we will call $ic_{\rm 0}$, is defined as $$ic_{\rm 0}   =  (uvw2-uvw1) - \frac{E(uvw2-uvw1)}{E({u-uvw1)}}  (u-uvw1)$$ where $E(x-y)$ stands for the colour excess of the ($x-y$) colour index. $ic_{\rm o}$ will remain a reddening free colour index as long as the colour excess ratio remains constant. This property is verified for stars with $T_{\rm eff}~\geq~9\,000$ K and $A_{\rm V}~\leq~5.5$. Nevertheless, since $A_{\rm 2000}~\sim~3~A_{\rm V}$, four magnitudes of absorption in $V$ correspond 