\begin{table}%t9
%\centering
\par
\caption{Observed abundance ratios assuming constant abundances.}
\small
\begin{tabular}{lrrrr} \hline\hline
Source & CN/HCN & CN/NO & CO$^+$/HCO$^+$ & SO$^+$/SO \\ \hline
AFGL 2591  & 1.15 & 0.74 & 0.016 & 0.006 \\
W3 IRS 5   & 1.80 & 0.21 & 0.066 & 0.007 \\
IRAS 16293 & 0.05 & ... & 0.001 &  $<$0.001 \\
N1333-I2   & 0.20 & 0.16 & 0.002 & $<$0.001 \\
N1333-I4A  & 0.12 & 0.03 & $<$0.002 & $<$0.001 \\
L1448-C    & 0.20 & ... & $<$0.002 &  $<$0.007 \\
L483       & 0.75 & ... & $<$0.004 & ... \\
L723       & 0.92 & ... & $<$0.005 & $<$0.017 \\
SMM4       & 0.08 & ... & $<$0.001 & ... \\
L1489      & 3.85 & ... & ... & ... \\ \hline
\label{tar}
\end{tabular}
\end{table}