\begin{table}%t8
\caption{\label{table:d_frac}N$_2$H$^+$ and N$_2$D$^+$ column densities, fractional abundances,
and the column density ratio.}
%\centerline
 {
\begin{tabular}{c c c c c c}
\hline\hline 
         & $N({\rm N_2H^+})$ & $N({\rm N_2D^+})$ & $x({\rm N_2H^+})$ & $x({\rm N_2D^+})$ & \\
Position & [$10^{12}$~cm$^{-2}$] & [$10^{11}$~cm$^{-2}$] & [$10^{-10}$] & [$10^{-11}$] & $R_{\rm deut} \equiv N({\rm N_2D^+})/N({\rm N_2H^+})$\\
\hline
IRAS~05405-0117 & $9.14\pm0.08$$^a$ & $3.19\pm0.37$ & 11.1 & 4.9 & $0.03\pm0.004$\\
Ori~B9~E & $4.54\pm0.65$ & $1.90\pm1.39$ & 6.9 & 5.0 & $0.04\pm0.03$\\
Ori~B9~N$^{b}$ & $4.11\pm1.51$ & $1.46\pm0.65$ & 3.9 & 1.9 & $0.04\pm0.02$\\
\hline 
\end{tabular}}
\medskip
\par
$^a$ Harju et~al. (2006) estimated 
slightly lower N$_2$H$^+$ column density, ${\sim}6{-}8\times 10^{12}$~cm$^{-2}$, toward IRAS~05405-0117 from the N$_2$H$^+(1{-}0)$ data of Caselli \& Myers (1994).
\\
$^{b}$ For the other velocity component $N({\rm N_2H^+})=1.56\pm0.09\times 10^{12}$~cm$^{-2}$, $N({\rm N_2D^+})=7.64\pm5.44\times 10^{10}$~cm$^{-2}$, and $R_{\rm deut}=0.05\pm0.03$.
\end{table}