Table 1: 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 M₁ and M₂, the mass ratio q₂ = M₂/M₁, the estimated cooling age of the youngest white dwarf $\tau _2$ and the difference between the cooling ages of the components $\Delta \tau$ . M₁ 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) Saffer et al. (1988); (2) Bergeron et al. (1989); (3) Bragaglia et al. (1990); (4) Marsh (1995); (5) Moran et al. (1997); (6) Moran et al. (1999); (7) Maxted et al. (2000); (8) Bergeron & Liebert (2002); (9) Maxted et al. (2002a); (10) Maxted et al. (2002b); (11) Napiwotzki et al. (2002); (12) Karl et al. (2003a); (13) Karl et al. (2003b). 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).
Name $P_{\rm orb}$ (d) $a_{\rm orb}$ ( $R_\odot$ ) M₁ ( $M_\odot$ ) M₂ ( $M_\odot$ ) q₂ = M₂/M₁ $\tau _2$ (Myr) $\Delta \tau$ (Myr) Ref/Note

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

WD 0136+768 1.407 4.98 0.37 0.47 1.26 $\pm$ 0.03 150 450 3, 10

WD 0957-666 0.061 0.58 0.32 0.37 1.13 $\pm$ 0.02 25 325 3, 5, 6, 10

WD 1101+364 0.145 0.99 0.33 0.29 0.87 $\pm$ 0.03 135 215 4, (10)

PG 1115+116 30.09 46.9 0.7 0.7 0.84 $\pm$ 0.21 60 160 8, 9

WD 1204+450 1.603 5.72 0.52 0.46 0.87 $\pm$ 0.03 40 80 6, 10

WD 1349+144 2.209 6.65 0.44 0.44 1.26 $\pm$ 0.05 - - 12

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

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

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

**Table 1:** 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 M₁ and M₂, the mass ratio q₂ = M₂/M₁, the estimated cooling age of the youngest white dwarf $\tau _2$ and the difference between the cooling ages of the components $\Delta \tau$ . M₁ 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) Saffer et al. (1988); (2) Bergeron et al. (1989); (3) Bragaglia et al. (1990); (4) Marsh (1995); (5) Moran et al. (1997); (6) Moran et al. (1999); (7) Maxted et al. (2000); (8) Bergeron & Liebert (2002); (9) Maxted et al. (2002a); (10) Maxted et al. (2002b); (11) Napiwotzki et al. (2002); (12) Karl et al. (2003a); (13) Karl et al. (2003b). 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).
Name	$P_{\rm orb}$ (d)	$a_{\rm orb}$ ( $R_\odot$ )	M₁ ( $M_\odot$ )	M₂ ( $M_\odot$ )	q₂ = M₂/M₁	$\tau _2$ (Myr)	$\Delta \tau$ (Myr)	Ref/Note
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
WD 0136+768	1.407	4.98	0.37	0.47	1.26 $\pm$ 0.03	150	450	3, 10
WD 0957-666	0.061	0.58	0.32	0.37	1.13 $\pm$ 0.02	25	325	3, 5, 6, 10
WD 1101+364	0.145	0.99	0.33	0.29	0.87 $\pm$ 0.03	135	215	4, (10)
PG 1115+116	30.09	46.9	0.7	0.7	0.84 $\pm$ 0.21	60	160	8, 9
WD 1204+450	1.603	5.72	0.52	0.46	0.87 $\pm$ 0.03	40	80	6, 10
WD 1349+144	2.209	6.65	0.44	0.44	1.26 $\pm$ 0.05	-	-	12
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
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
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

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