\begin{table}%t5
\caption{\label{D/H}N$_2$H$^+$ and N$_2$D$^+$ column densities.} 
\small%\centerline
{
\begin{tabular}{lccc}
\hline\hline
  Source & $N$(N$_2$H$^+$) & $N$(N$_2$D$^+$) & $N$(N$_2$D$^+$)/$N$(N$_2$H$^+$) \\
  & [cm$^{-2}$] & [cm$^{-2}$] \\
\hline \noalign{\smallskip}
L 1448 IRS 2 &$3.1 \times 10^{13}$ &$1.3 \times 10^{12}$ & 0.042~$\pm$~0.014\\
L 1448 IRS 3 &$4.8 \times 10^{13}$ &$3.9 \times 10^{12}$ & 0.081~$\pm$~0.010\\
L 1448 C & $2.4 \times 10^{13}$ &$2.5 \times 10^{12}$ &0.104~$\pm$~0.017\\
L 1455 A1 &$9.2 \times 10^{12}$ &$6.3 \times 10^{11}$ & 0.068~$\pm$~0.043\\
L 1455 A2 &$1.4 \times 10^{13}$ &$9.6 \times 10^{11}$ & 0.069~$\pm$~0.029\\
Per 4 S &$7.9 \times 10^{12}$ &$1.4 \times 10^{12}$ & 0.177~$\pm$~0.032\\
Per 4 N &$7.1 \times 10^{12}$ &$7.8 \times 10^{11}$ & 0.110~$\pm$~0.057\\
Per 5 & $1.2 \times 10^{13}$ &$1.0 \times 10^{12}$ & 0.083~$\pm$~0.033\\
Per 6 & $6.1 \times 10^{12}$ & $<$$1.8\times 10^{11}$ & $<$0.029\\
IRAS 03282 & $1.6 \times 10^{13}$ &$3.5 \times 10^{12}$ & 0.219~$\pm$~0.026 \\
Barnard 1-B &$4.5 \times 10^{13}$ &$8.1 \times 10^{12}$ & 0.180~$\pm$~0.009\\
HH 211 &   $1.7\times 10^{13}$ & $4.6\times 10^{12}$ & 0.271~$\pm$~0.024\\
Barnard 5 IRS 1 &  1.$6\times 10^{13}$ & $5.4\times 10^{11}$ & 0.034~$\pm$~0.025\\
L 1527 & $7.2 \times 10^{12}$ & $<$$ 2.5\times 10^{11}$ & $<$0.034\\
L 483 &  $3.3 \times 10^{13}$ &$1.9 \times 10^{12}$ & 0.057~$\pm$~0.012\\
SMM 5 &  $2.9 \times 10^{13}$ &$2.3 \times 10^{12}$ & 0.079~$\pm$~0.019  \\
L 723 &  $8.8 \times 10^{12}$ &  $4.0 \times 10^{11}$ & 0.045~$\pm$~0.040  \\
L 673 A &  $ 1.3\times 10^{13}$ &$4.3 \times 10^{11}$ & 0.033~$\pm$~0.031   \\
B 335 &  $1.4 \times 10^{13}$ &$6.0 \times 10^{11}$ & 0.043~$\pm$~0.029     \\
L 1157 & $1.1 \times 10^{13}$ &$5.0 \times 10^{11}$ & 0.045~$\pm$~0.036   \\
\hline
\end{tabular}}
\smallskip
Estimated errors on the column densities are about 15\% and 20\% for N$_2$H$^+$ and N$_2$D$^+$, respectively.
\end{table}