\begin{table}%t3
\par
\caption{\label{tabla-he}$^4$He observational abundances $(Y_{^4{\rm He}}^{\rm obs})$.}
%\centering
\par
\begin{tabular}{lc}
\hline \hline\noalign{\smallskip}
$Y_{^4{\rm He}}^{\rm obs} \pm \sigma_{^4{\rm He}}^{\rm obs}$ & Refs. \\ 
\hline
$ 0.2565 \pm 0.0010$ &\citet{izotov10} \\
$ 0.2561 \pm 0.0108$ & \citet{aver10}  \\
\hline
\end{tabular}
\end{table}