\begin{table}%t6
\caption {\label{table:MLrelations}$ML$ relations for the selected  cases; $\log (L/L_{\odot}) = a + b\; \log (M/M_{\odot})$.}   
%\centering
\par
\begin{tabular}{l l l l }       
\hline\hline
Case & $a$ & $b$ & $\sigma$\\  
\hline
I  ($std$)      & 1.000 $ \pm $ 0.017 & 3.160  $ \pm $ 0.019 & 0.009 \\
II ($Z_{\rm low}$)     & 1.040 $ \pm $ 0.010 & 3.110  $ \pm $ 0.011 & 0.008 \\
III ($Z_{\rm high}$)     & 0.736 $ \pm $ 0.008& 3.455  $ \pm $ 0.010 & 0.002 \\
IV ($Y_{\rm high}$)      & 1.095 $ \pm $ 0.043 & 3.074  $ \pm $ 0.047 & 0.024 \\
\hline                           
\end{tabular}
\end{table}