\begin{table}%t2
%\centering
\par
\caption{Photometric data of S~23 (in mag) collected from the literature
and sorted by their year(s) of observation.}\label{photo}
  \begin{tabular}{lcccccccl}
    \hline
    \hline
 Year &  $U$ & $B$ & $V$ & $R$ & $J$ & $H$ & $K/K_{\rm s}$ & References \\
    \hline
 1972--74 & 13.27 & 13.43 & 13.37 & -- & -- & -- & -- & Azzopardi \& Vigneau (\cite{Azzop82})  \\
 1988    & -- & -- & -- & -- & 13.04 & 12.79 & 12.16 & Zickgraf et~al. (\cite{Zick92}) \\
 1998    & -- & -- & -- & -- & 13.02 & 12.76 & 12.15 & Skrutskie et~al. (\cite{Skrutskie2006}, 2MASS) \\
 1999    & 13.11 & 13.38 & 13.31 & 13.19 & -- & -- & -- & Massey (\cite{Massey02}) \\
    \hline
  \end{tabular}
\end{table}