\begin{table}%t1
\caption{\label{tab:tauffall}Optical depth at various radio frequencies, for the four sources.}
%%\centering

\par
\begin{tabular}{lccc}
\hline \hline \noalign{\smallskip}
\multirow{2}{*}{Source} &\multicolumn{3}{c}{$\tau_{\rm ff}$}\\
&1.4 GHz&6.5 GHz& 8.5 GHz\\
\hline
\otto&2.2&1.2&0.05\\
\sette&27.8&1.1&0.7\\
\nove&5&0.2&0.1\\
\hline
&1.4 GHz&5 GHz& 6 GHz\\
\sax&9.3&0.7&0.5\\
\hline
\end{tabular}
\end{table}