  A population number synthesis code follows in detail the evolution of a
population of single stars and of close binaries. We use our code to simulate
the population of neutron star - neutron star and black hole - neutron star
binaries. We then combine our population number synthesis code with a galactic
chemical evolutionary model in order to follow the time evolution of the
formation and merger rate of these double compact star binaries and the
resulting chemical enrichment of r-process elements, over the whole Galactic
lifetime. It can be concluded that the neutron star/black hole merger process
is able to reproduce the observed r-process enrichment of the Galaxy. However,
we show that the latter conclusion depends critically on the physics of case BB
Roche lobe overflow in binaries with a neutron star component and a hydrogen
deficient core helium/helium shell burning star with a mass between 2.6 Mo and
6 Mo.
