\begin{table}%T1
\caption{Orbital parameters of 4U~1538-52.}\label{t1}
%\centerline
{\small
\begin{tabular}{c | c c} \hline\hline
Parameter &  Elliptical orbit & Circular orbit \\ \hline
$T_{\pi/2}$ Orbital epoch (MJD) & 52855.0421 $\pm$ 0.025 & 52855.0441 $\pm$ 0.025 \\
$a_{x}$ $\sin$ $i$ (lt-s)$^{a}$ & 56.6 $\pm$ 0.7 & 54.3 $\pm$ 0.6 \\
$e^{a}$                          & 0.174 $\pm$ 0.015   &              \\
$\omega$$^{a}$(deg)              & 64 $\pm$ 9         &               \\
$P_{\rm orbit}$$^{a}$              &3.7228366 $\pm$ 0.000032 &3.7228366 $\pm$ 0.000032      \\
Epoch (MJD)     & 52855.0585 $\pm$ 0.025 &52855.0585 $\pm$ 0.025  \\
$P_{\rm pulse}$ (s) & 526.8551 $\pm$ 0.016 &  526.8535 $\pm$ 0.013 \\
$\dot \nu$ (Hz s$^{-1}$)  &  $(2.838\pm4.124)\times 10^{-13}$ & 
$(2.241\pm 2.764)\times 10^{-13}$ \\
reduced $ \chi^{2}$  & 1.44 & 1.0 \\ \hline
\end{tabular}}
\par
$^{a}$ Taken from Clark (\cite{Clark00}).
\end{table}