\begin{table}%t6
   \caption{Calculated fractional abundances, with respect to $n$(H$_2$).\label{fracabunds}}
%\begin{center}
{  \begin{tabular}{llc cccccc cccccc}
%
   \hline\hline
&& && & \object{IRAS~16594--4656} & &&  &\object{IRAS~17150--3224} & \\
\cline{5-7}    \cline{9-11}
Molecule     && Transition          &&$r_{\rm i}$&$r_{\rm e}$& $f_{\rm X}$            &&$r_{\rm i}$&$r_{\rm e}$& $f_{\rm X}$\\
             &&                     && cm        & cm        &              && cm        & cm        &  \\
    \hline
$^{13}$CO$^a$&&	--      && --       & 2.9~(17) &  3.3~(--5) && --& 2.4~(17) &2.9~(--5)\\
HCN	     &&	$J=1$--0&& --       & 2.3~(16) &1.0~(--6)&& --       & 4.4~(16) &3.8~(--7)\\
HCN	     &&	$J=3$--2&& --       & 2.3~(16) &1.9~(--7)&& --       & 4.4~(16) &--\\
CN	     &&	$N=1$--0&& 1.7~(16) & 5.1~(16) &9.6~(--6)&& 3.9~(16) & 9.8~(16) &U.L.$^b$\\
CN	     &&	$N=2$--1&& 1.7~(16) & 5.1~(16) &2.5~(--6)&& 3.9~(16) & 9.8~(16) &U.L.$^b$\\
  \hline\\[-3mm]
   \end{tabular}}
\parbox{16cm}{
        $^a$ $^{13}$CO  fractional abundances derived from full radiative transfer (Sect.~\ref{modelling}).\\
        $^b$ Signifies a non-detection, but the calculation of an upper limit to the fractional abundance has been made (see Table~\ref{upperlimits}).}
%\end{center}
\par
\end{table}