\begin{table}%t6
\caption{\label{table6}Average isotopic ratios. The weighted mean is computed for a variety 
of groups:  all data, all data excluding 73P/Schwassmann-Wachmann~3~B and~C, and each of the categories of Levison (JF, HF, EXT, NEW) or Horner et~al. (SP, I, L).}
%\centering
\small
\begin{tabular}{lll} 
\hline \hline 
%&&& \\[-8pt]
Group& $^{12}$C/$^{13}$C &  $^{14}$N/$^{15}$N  \\
\hline
All comets&$91.1\pm3.7$&$147.8\pm5.6$\\
All without S-W 3&$90.6\pm3.8$&$145.2\pm5.6$\\
JF &$97.2\pm7.6$&$156.8\pm12.2$\\
JF without S-W&$96.3\pm8.6$&$146.4\pm12.4$\\
Non JF (Oort)&$89.1\pm4.2$&$144.0\pm6.5$\\
HF&$90.0\pm8.4$&$146.5\pm15.1$\\
EXT &$89.0\pm6.9$&$141.3\pm10.7$\\
NEW &$89.1\pm6.8$&$145.3\pm9.5$\\
SP$_{\rm I}$&$90.0\pm8.4$&$146.5\pm15.1$\\
SP$_{\rm III}$&$100.0\pm15.1$&$216.1\pm27.3$\\
SP$_{\rm IV}$&$96.3\pm8.6$&$146.4\pm12.4$\\
I&$88.2\pm9.3$&$142.8\pm14.8$\\
L&$89.3\pm5.8$&$143.8\pm8.1$\\
\hline
\end{tabular}
\end{table}