\begin{table}%t1
%\centering
\par
\caption{Parameters of the observed transitions used to study the PDRs
in Cepheus~B.
\label{tab_exc}}
 \begin{tabular}{llll}
\hline
\hline
Species & Transition & $E_{\rm u}$/k & $n_{\rm cr}$ \\
&& (K) &  (\cmcub)\\
\hline\noalign{\smallskip}
\CI\ & $^3$P$_1$ $\rightarrow$ $^3$P$_0$ & 23.6 & $1\times10^3$\\
\twCO\  & 1--0 & 5.6 & $4.8\times10^3$ \\
\thCO\  & 2--1 & 16.8 & $3.2\times10^4$ \\
\twCO\ &  4--3 & 56  & $2.6\times10^5$ \\
\hline
\end{tabular}
\end{table}