\begin{table}%t5
\caption{\label{NH_AV_values}The $N_{\rm H}$/$A_{V}$ ratios from the 
literature (see also Table~\ref{NH_AV_ratio}).}
%\centerline
{
\begin{tabular}{cc}
\hline
\hline
$N_{\rm H}$/$A_{V}$  & Ref.\\
\hline
$1.8 \times10^{21}$ cm$^{-2}$ $^a$ & \citet{BOH78} \\
$1.9 \times10^{21}$ cm$^{-2}$ $^a$ & \citet{WHI81} \\
$2.2 \times10^{21}$ cm$^{-2}$ $^{a,b}$ & \citet{GOR75, RYT96} \\
$2.0 \times10^{21}$ cm$^{-2}$ $^b$ & \citet{RYT75} \\
$1.8 \times10^{21}$ cm$^{-2}$ $^b$ & \citet{PRE95}\\
\hline\end{tabular}}
\smallskip
$^a$ UV; $^b$ X-rays. 
\end{table}