\begin{table}%a1
%\centering
\par
\caption{Column densities $N({\rm CO^+})$ and fractional abundances 
$x({\rm CO^+})$ calculated with Eq. (\ref{en}) assuming $T_{\rm ex}({\rm CO^+}) 
= 20$~K. The upper limits correspond to $3\sigma$ in line flux.}
\begin{tabular}{lrr} \hline\hline
Source     & $N({\rm CO^+})$ & $x({\rm CO^+})$ \\
           & $10^{12}$~cm$^{-2}$ &   \\ \hline \noalign{\smallskip}
AFGL 2591  &    1.0 & $1.0\times 10^{-11}$ \\
W3 IRS 5   &    1.7 & $2.5\times 10^{-12}$ \\
IRAS 16293 &    0.6 & $3.9\times 10^{-13}$ \\
N1333-I2   &    0.2 & $3.3\times 10^{-13}$ \\
N1333-I4A  & $<$$0.1$ & $<$$6.0\times 10^{-14}$ \\
L1448-C    & $<$$0.1$ & $<$$6.0\times 10^{-13}$ \\
L483       & $<$$0.1$ & $<$$9.0\times 10^{-14}$ \\
L723       & $<$$0.1$ & $<$$3.0\times 10^{-13}$ \\
SMM4       & $<$$0.1$ & $<$$9.0\times 10^{-14}$ \\
V1057 Cyg  & $<$$0.1$ & $<$$2.0\times 10^{-12}$ \\ \hline
\label{tcol}
\end{tabular}
\begin{list}{}{}
\item[]
\end{list}
\end{table}