Extending the population synthesis method to isolated young cooling white dwarfs we are able to confront our model assumptions with observations made in ROSAT All-Sky Survey [ ] .
This allows us to check model parameters such as evolution of spectra and separation of heavy elements in DA WD envelopes .
It seems like X-ray spectrum temperature of these objects is given by the formula [ ] T _ { X \mathchar 45 \relax ray } ~ { } = ~ { } \min \left ( T _ { eff } ,T _ { max } \right ) .
We have obtained DA WD ’ s birth rate and upper limit of the X-ray spectrum temperature : DA birth rate = 0.61 \times 10 ^ { -12 } { pc ^ { -3 } yr ^ { -1 } } , T _ { max } = 4.1 \times 10 ^ { 4 } { K } .
These values are in good correspondence with values obtained by [ ] .
From this fact we also conclude that our population synthesis method is applicable to the population of close-by isolated cooling white dwarfs as well as to the population of the isolated cooling neutron stars .