is necessary in order to calculate the formation rates of low-mass X-ray binaries (LMXBs) and $\gamma$-ray bursts, as well as for LISA and LIGO sources [@Bel08]. Understanding which evolutionary channels are viable pathways for the creation of double-degenerate systems such as double white dwarfs (DWDs) is one of the key ‘sanity checks’ available in studying binary evolution, providing tight observational constraints for the otherwise almost entirely theoretical study of the evolution of multiple stars.

The majority of currently observed DWDs consist of relatively low-mass helium dwarfs [@SPY01; @SPY02; @SPY03; @SPY04; @SPY05]. The observed DWDs for which the older companion is less massive than $0.46\,M_\odot$ have an average orbital period of 0.96d and an average mass for the (inferred) younger companion of $0.39\,M_\odot$ [based on the double-lined spectroscopic binaries observed prior to this year, @Sluys2006 see table 1].

In order to form WD companions of the observed masses, a DWD system must necessarily have undergone two episodes of envelope mass loss as each component went through the giant phase. This can be inferred from the fact that none of these observed WDs are sufficiently massive to have evolved independently to a degenerate state within a Hubble time, and so their progenitors, 