Simulated population of visible double white dwarfs as a function of orbital period and the combined mass of the two dwarfs. On the left the common envelope is parametrised according to model γα, on the right according to model αα (see Sect. 3). The intensity of the grey scale corresponds to the density of objects on a linear scale. The same grey scale is used for both plots. Observed binary white dwarfs are overplotted with filled circles, see Fig. 1 for references. The Chandrasekhar mass limit is indicated with the dotted line. The dashed line roughly demarks the region in which systems merge within a Hubble time. Systems located to the left of the dashed line and above the dotted line are supernova type Ia progenitors in the standard picture.