\begin{table}%T6
\caption{\label{PRlett}Comparison between the PR ($\log R = a \log P +b$) coefficients 
obtained in this paper and the ones based on selected works in literature. 
In particular, from left to right, we report the slope, 
the zero point, the source and the method adopted for deriving the~PR.}  
%\centerline
{
\begin{tabular}{l l l l l}
\hline
\hline
$a$&$b$&Source&Method \\ 
\hline         
0.74~$\pm$ 0.03&1.12~$\pm$ 0.03&RAAF   &Surf. brightness \\
0.751~$\pm$ 0.026&1.070~$\pm$ 0.008&LS &Surf. brightness \\
0.750~$\pm$ 0.024&1.075~$\pm$ 0.007&GFG&Surf. brightness \\
%0.622~$\pm$ 0.029&1.226~$\pm$ 0.026&RBMR&CORS (senza~$\Delta B$) \\
0.606~$\pm$ 0.037&1.263~$\pm$ 0.033&RBMR&CORS  \\
0.655~$\pm$ 0.006&1.188~$\pm$ 0.008&BCM&Theory \\ 
 0.69~$\pm$ 0.09& 1.18~$\pm$ 0.08&This work & new CORS (without~$\Delta B$) \\
 0.74~$\pm$ 0.11& 1.19~$\pm$ 0.09&This Work & new CORS (with~$\Delta B$) \\
\hline
\end{tabular}}
\smallskip
PR source: Rojo~Arellano \& Arellano Ferro (\cite{Rojo1994}, RAAF); Laney \& Stobie (\cite{Laney1995}, LS); 
Gieren et~al. (\cite{Gieren1998}, GFG); Ripepi et~al. (\cite{Ripepi1997}, RBMR); 
\mbox{Arellano~Ferro} \& Rosenzweig (\cite{Arellano2000}, AFR); Bono et~al. (\cite{Bono1998}, BCM). 
\end{table}