  We carried out a spectroscopic investigation of single lined white dwarfs
(WDs) in double degenerate (DD) systems and discuss their binary evolution.
Simulated spectra of the Halpha region are used to derive upper limits on the
temperature of the invisible component and thus lower limits on the cooling
age. This is done for a range of hypothetical secondary masses and a minimum
cooling age deduced. Results are compared with the well known parameters of the
visible primary, which allows us to determine a lower limit for the cooling age
difference of both WDs. Most of the ten systems in our sample have a minimum
age difference of not larger than 0.5Gyr and their small orbital separation is
highly suggestive of at least one unstable mass transfer phase. However, a
stable first mass transfer phase is feasible as the age difference is less then
1Gyr. The results imply that unstable mass transfer is the most likely final
contact binary scenario to have occurred in DD systems but the first mass
transfer phase is not constrained.
