The age of white dwarf companions
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 Hα 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 0.5 Gyr 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 1 Gyr. 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.