\begin{table}%t1
\caption{Observed double white dwarfs discussed in this paper. The table shows for each system the orbital period $P_{\rm orb}$,
the orbital separation~$a_{\rm orb}$, the masses $M_1$ and $M_2$, the mass ratio $q_2$ = $M_2/M_1$, 
the estimated cooling age of the youngest white dwarf $\tau_2$ and the difference between the cooling ages of the 
components $\Delta\tau$. $M_1$ is the mass of the oldest white dwarf and 
thus presumably the original primary. The errors on the periods are smaller than the last digit.  The values for $a_{\rm orb}$ are 
calculated by the authors and meant to give an indication.  
References: 
(1)~\citet{1988ApJ...334..947S};
(2)~\citet{1989ApJ...345L..91B};
(3)~\citet{1990ApJ...365L..13B};
(4)~\citet{1995MNRAS.275L...1M};
(5)~\citet{1997MNRAS.288..538M};
(6)~\citet{1999ASPC..169..275M};
(7)~\citet{2000MNRAS.314..334M};
(8)~\citet{2002ApJ...566.1091B};
(9)~\citet{2002MNRAS.334..833M};
(10)~\citet{2002MNRAS.332..745M}; 
(11)~\citet{2002AetA...386..957N}; 
(12)~\citet{2003whdw.conf...43K}; 
(13)~\citet{2003AetA...410..663K}.
Note: (a) WD~1704+481a is the close pair of a hierarchical triple.  It seems unclear which of the two stars in this pair is the youngest 
(see the text).
\label{tab:obswds}  }
\small%\centering
\par
\begin{tabular}{lllllllll}
\hline \hline                                                                                        Name             & $P_{\rm orb}$ (d)  & $a_{\rm orb}$ ($R_\odot$)    & $M_1$ ($M_\odot$) & $M_2$ ($M_\odot$) & $q_2$ = $M_2/M_1$ & $\tau_2$ (Myr) & $\Delta \tau$ (Myr) & Ref/Note \\
\hline
\object{WD~0135--052}     & 1.556      & 5.63             & 0.52 $\pm$ 0.05   & 0.47 $\pm$ 0.05   & 0.90 $\pm$ 0.04  &  950 & 350 &  1, 2  \\ 
\object{WD~0136+768}      & 1.407      & 4.98             & 0.37		     & 0.47 	         & 1.26 $\pm$ 0.03  &  150 & 450 &  3, 10  \\ 
\object{WD~0957--666}     & 0.061      & 0.58             & 0.32		     & 0.37 	         & 1.13 $\pm$ 0.02  &   25 & 325 &  3, 5, 6, 10  \\ 
\object{WD~1101+364}      & 0.145      & 0.99             & 0.33		     & 0.29 	         & 0.87 $\pm$ 0.03  &  135 & 215 &  4, (10)  \\ 
\object{PG~1115+116}      & 30.09      & 46.9             & 0.7		     & 0.7               & 0.84 $\pm$ 0.21  &   60 & 160 &  8, 9   \\
\multicolumn{9}{c}{}\\
\object{WD~1204+450}      & 1.603      & 5.72             & 0.52		     & 0.46 	         & 0.87 $\pm$ 0.03  &   40 & 80  &  6, 10  \\ 
\object{WD~1349+144}      & 2.209      & 6.65             & 0.44		     & 0.44 	         & 1.26 $\pm$ 0.05  &   --  & --   &  12  \\ 
\object{HE~1414--0848}    & 0.518      & 2.93             & 0.55 $\pm$ 0.03   & 0.71 $\pm$ 0.03   & 1.28 $\pm$ 0.03  & 1000 & 200 &  11   \\ 
\object{WD~1704+481a}     & 0.145      & 1.13             & 0.56 $\pm$ 0.07   & 0.39 $\pm$ 0.05   & 0.70 $\pm$ 0.03  &  725 & --20 &  7, a \\ 
\object{HE~2209--1444}    & 0.277      & 1.89             & 0.58 $\pm$ 0.08   & 0.58 $\pm$ 0.03   & 1.00 $\pm$ 0.12  &  900 & 500 &  13   \\
\hline
\end{tabular}
\end{table}