\begin{table}%t3
%\centering
\par
\caption{Apsidal motion elements of V539 Ara, GG~Lup, V526 Sgr and AO~Vel.}
\label{t2}
\begin{tabular}{llllll}
\hline\hline\noalign{\smallskip}
Element      &   Unit  &  V539 Ara & GG Lup  & V526 Sgr & AO Vel \\
\hline\noalign{\smallskip}
$T_0$ & HJD  & 24 45056.7459 (7) & 24 46136.7448 (1)  
             & 24 47739.6517 (2) & 24 45043.6700 (3) \\
$P_{\rm s}$ & days &  3.1690854 (12)  & 1.8495996 (15) 
             &  1.91941177 (8)  & 1.5846212 (7) \\
$P_{\rm a}$ & days &  3.1692552 (12)  & 1.8496919 (15) 
             &  1.91947680 (8)  & 1.5847476 (7) \\
$e$   &      &  0.0548 (15) & 0.1546 (10) & 0.2204 (4) & 0.0735 (6)  \\
$\dot{\omega}$ & deg $\:\rm{cycle^{-1}}$ & 0.0193 (10) & 0.0180 (5) 
                                         & 0.01219 (3) & 0.0287 (3)  \\
$\dot{\omega}$ & deg $\:\rm{yr^{-1}}$    & 2.22 (0.1)   & 3.54 (10) 
                                         & 2.320 (6)    & 6.61 (10) \\
$\omega_0$   & deg                       & 118.9 (1.2)  & 86.12 (0.15) 
                                         & 254.9 (0.1)  & 97.1 (0.1) \\
$U$          & years                     & 161.8 (8.1)  & 101.6 (1.0) 
                                         & 155.2 (0.2)  & 54.4 (0.5) \\
\hline\noalign{\smallskip}
\end{tabular}
\end{table}