We use the well tested StarTrack binary population synthesis code to examine the properties of the population of compact object binaries .
We calculate the distribution of masses and mass ratios taking into account weights introduced by observability in gravitational waves during inspiral .
We find that in the observability weighted distribution of double neutron star binaries there are two peaks : one for nearly equal mass systems , and one for the systems consisting of a low and a high mass neutron star , q = 0.6 - 0.7 .
The observability weighted distribution of black hole neutron star binaries is concentrated on the systems with the mass ratio q = 0.3 - 0.5 , while for the double black hole binaries the observability weighted distribution is dominated by the massive , nearly equal mass binaries with q > 0.7 .