You can observe hundreds of pulsars which are neutron stars, it is very difficult to observe the mass of the neutron star because you need a binary for that purpose. So Gerry Brown speculated about the way a binary of neutron stars could be made. And he found out that this is indeed very difficult because if you make a neutron star in the neighborhood of a... another big star, then this other big star would have a.. the halo which goes very far out, which contains matter and this matter would accrete to the neutron star and would bring it over the limit, make it a black hole and make it disappear. And the... well, he used work by van der Heel [sic] of Holland who had studied this matter in great detail and Gerry Brown concluded after long study that there was really only one way to... to make a binary of neutron stars, and that is you first have to have a binary of stars of approximately equal mass. Then in the course of time these stars would each evolve into a white dwarf and you would get then white dwarfs of approximately equal mass, and that's only possible that... that's only possible if the two stars have very nearly the same mass from the beginning.