We model the population of double white dwarfs in the Galaxy and find a better agreement with observations compared to earlier studies , due to two modifications .
The first is the treatment of the first phase of unstable mass transfer and the second the modelling of the cooling of the white dwarfs .

A satisfactory agreement with observations of the local sample of white dwarfs is achieved if we assume that the initial binary fraction is \sim 50 % and that the lowest mass white dwarfs ( M < 0.3 \mbox { $ { M } _ { \sun } $ } ) cool faster than the most recently published cooling models predict .

With this model we find a Galactic birth rate of close double white dwarfs of 0.05 yr ^ { -1 } , a birth rate of AM CVn systems of 0.005 yr ^ { -1 } , a merger rate of pairs with a combined mass exceeding the Chandrasekhar limit ( which may be progenitors of SNe Ia ) of 0.003 yr ^ { -1 } and a formation rate of planetary nebulae of 1 yr ^ { -1 } .
We estimate the total number of double white dwarfs in the Galaxy as 2.5 \times 10 ^ { 8 } .

In an observable sample with a limiting magnitude V _ { lim } = 15 we predict the presence of \sim 855 white dwarfs of which \sim 220 are close pairs .
Of these 10 are double CO white dwarfs of which one has a combined mass exceeding the Chandrasekhar limit and will merge within a Hubble time .