\begin{table}%t1
  \caption{Relative astrometry and photometry for the four components
    of \obj\  and for the lensing galaxies~G1 and~G2.}\label{astr_HST}
    %\centering
\small
  \begin{tabular}{c|ccc}
    \hline
    \hline
    ID & $\Delta \alpha$ (\arcsec) & $\Delta \delta$ (\arcsec) & Magnitude \\ 
    \hline
    $B$  &  0.                    &  0.                   & 17.77 $\pm$ 0.02 \\
    $A1$ & $-2.1946$ $\pm$ 0.0004 &  1.2601  $\pm$ 0.0003 & 17.16 $\pm$ 0.02 \\
    $A2$ & $-1.4809$ $\pm$ 0.0004 &  1.3756  $\pm$ 0.0005 & 17.52 $\pm$ 0.02 \\
    $C$  & $-2.1128$ $\pm$ 0.0003 &$-0.2778$ $\pm$ 0.0003 & 17.88 $\pm$ 0.02 \\
    G1 & $-1.4388$ $\pm$ 0.0019 &  0.3113  $\pm$ 0.0008 & 18.59 $\pm$ 0.03 \\
    G2 & $-5.4100$ $\pm$ 0.0006 &  0.2850  $\pm$ 0.0003 & 18.14 $\pm$ 0.02 \\
    \hline
  \end{tabular}
\end{table}