\begin{table}%t2
%\centering
\par
\caption{\label{tab:YZmix}$Y$ and $Z$ values for the three different adopted mixtures (see text   in Sect.~\ref{subsec:mixt}),
for two reference values of [Fe/H] and $\Delta Y/ \Delta Z = 4$. }
\begin{tabular}{cccc}
\hline \hline \noalign{\smallskip}
    & GN93   &  AGS05  & AGSS09 \\
    &  $\left(\frac{Z}{X}\right)_{\sun} =  0.0245$  & $\left(\frac{Z}{X}\right)_{\sun} =  0.0165$ & $\left(\frac{Z}{X}\right)_{\sun} =  0.0181$  \\[6pt]
\hline
\multirow{2}{*}{[Fe/H] = --0.6} & $Y = 0.2660$     & $Y = 0.2602$    & $Y =0.2614$ \\
& $Z = 0.00449 $ & $Z = 0.00305 $ & $Z = 0.00334$ \\ 
\hline
\multirow{2}{*}{[Fe/H] = 0.2} & $Y = 0.3458$  & $Y = 0.3178$  & $Y =0.3235$ \\
                                                & $Z = 0.0245 $ & $Z = 0.0174 $ & $Z = 0.0189$ \\ 
\hline
\end{tabular}
\end{table}