\begin{table}%t6
\caption{\label{tabella6}Absolute magnitudes $M_R$ of sample sources calculated adopting~$R1$ values from USNO-B1 (Table~\ref{tabella1}) and rest-frame peak frequencies~$\nu_{\rm s}$ adopting values relative to the faint state, as reported in~Table~\ref{tabella4}.}
%\centering
\par
\begin{tabular}{cccc}
\hline\hline \noalign{\smallskip}
Source & $m-M$ & $M_R$ & $\nu_{\rm s}$  \\
\hline
\a     & --42.3 & --24.8 & 13.28  \\
\b     & --43.0 & --23.3 & 13.33  \\
\c     & --44.6 & --25.9 & 13.18  \\
\d     & --42.3 & --23.1 & 13.04  \\
\e     & --42.3 & --23.1 & 12.96  \\
\f     & --42.3 & --24.0 & 13.12  \\
BZB~J1927+6117    & --42.3 & --25.3 & 12.94  \\
\l     & --42.3 & --23.3 & 13.25  \\
\hline
\end{tabular}
\end{table}