\begin{table}%T4
\caption{\label{tab:ct}Coefficients and range of applicability of the
  colour-temperature-metallicity relations.}
\small%\centering
\par
\begin{tabular}{lcccccccccc}
\hline\hline\noalign{\smallskip}
Colour    & $\feh$ range & Colour range  & $a_0$    & $a_1$    & $a_2$     & $a_3$     & $a_4$ & $a_5$ & $N$ & $\sigma (\teff)$ \\     
\hline
$B-V$     & $[-5.0,0.4]$ & $[0.18,1.29]$ & $0.5665$ & $0.4809$ & $-0.0060$ & $-0.0613$  & $-0.0042$ & $-0.0055$ & 400 & 73\\     
$V-R_{\rm C}$   & $[-5.0,0.3]$ & $[0.24,0.80]$ & $0.4386$ & $1.4614$ & $-0.7014$ &  $-0.0807$ & $0.0142$  & $-0.0015$ & 201 & 62\\
$(R-I)_{\rm C}$ & $[-5.0,0.3]$ & $[0.23,0.68]$ & $0.3296$ & $1.9716$ & $-1.0225$ & $-0.0298$  & $0.0329$  & $0.0035$  & 211 & 82\\
$V-I_{\rm C}$   & $[-5.0,0.3]$ & $[0.46,1.47]$ & $0.4033$ & $0.8171$ & $-0.1987$ & $-0.0409$  & $0.0319$  & $0.0012$  & 208 & 59\\
$V-J$     & $[-5.0,0.4]$ & $[0.61,2.44]$ & $0.4669$ & $0.3849$ & $-0.0350$ & $-0.0140$  & $0.0225$  & $0.0011$  & 401 & 42\\
$V-H$     & $[-5.0,0.4]$ & $[0.67,3.01]$ & $0.5251$ & $0.2553$ & $-0.0119$ & $-0.0187$  & $0.0410$  & $0.0025$  & 401 & 33\\
$V-K_{\rm S}$   & $[-5.0,0.4]$ & $[0.78,3.15]$ & $0.5057$ & $0.2600$ & $-0.0146$ &  $-0.0131$ & $0.0288$  & $0.0016$  & 401 & 25\\
$J-K_{\rm S}$   & $[-5.0,0.4]$ & $[0.07,0.80]$ & $0.6393$ & $0.6104$ & $0.0920$  & $-0.0330$  & $0.0291$  & $0.0020$  & 412 & 132\\
$(B-V)_{\rm T}$ & $[-2.7,0.4]$ & $[0.19,1.49]$ & $0.5839$ & $0.4000$ & $-0.0067$ & $-0.0282$  & $-0.0346$ & $-0.0087$ & 251 & 79\\
$V_{\rm T}-J$   & $[-2.7,0.4]$ & $[0.77,2.56]$ & $0.4525$ & $0.3797$ & $-0.0357$ & $-0.0082$  & $0.0123$  & $-0.0009$ & 272 & 43\\
$V_{\rm T}-H$   & $[-2.7,0.4]$ & $[0.77,3.16]$ & $0.5286$ & $0.2354$ & $-0.0073$ &  $-0.0182$ & $0.0401$  & $0.0021$  & 263 & 26\\
$V_{\rm T}-K_{\rm S}$ & $[-2.4,0.4]$ & $[0.99,3.29]$ & $0.4892$ & $0.2634$ & $-0.0165$ & $-0.0121$  & $0.0249$  & $-0.0001$ & 258 & 18\\
$b-y$     & $[-3.7,0.5]$ & $[0.18,0.72]$ & $0.5796$ & $0.4812$ & $0.5747$  & $-0.0633$  & $0.0042$  & $-0.0055$ & 1120 & 62 \\
\hline 
\end{tabular}
\tablefoot{The photometric systems are Johnson-Cousins $BV(RI)_{\rm C}$, 2MASS $JHK_{\rm S}$, Tycho2 $(BV)_{\rm T}$ and Str\"omgren $by$. For the latter, additional corrections as 
function of $\feh$ and $(b-y)$ apply, as discussed in Sect.~\ref{stromgren}. 
For some indices the calibrations are given down to $\feh=-5.0$, meaning that 
the effective temperatures of such a metal-poor star can be recovered using 
$\feh=-3.5$ in Eq.~(\ref{eq:ct}). Notice that only two hyper metal-poor stars 
are currently known and caution should be used, as discussed in the text. 
Especially for metal-poor stars, please refer to Fig.~\ref{f:space} to check 
that the calibration is not extrapolated outside its $\feh$ range.
$N$ is the number of stars employed for the fit after the $3 \sigma$
clipping and $\sigma(\teff)$ is the standard deviation (in Kelvin) of the
proposed calibrations. Notice that the standard deviation does not account for 
the uncertainty in the zero point of the temperature scale, which is of order 
$15{-}20$~K (Sects.~\ref{tuning} and~\ref{inter}).}
\end{table}