Mergers of binary compact objects

We work out the effects of hypercritical accretion, which transfers mass from the secondary to the primary (older) neutron star (NS) in a binary, showing that such accretion would cause the mass of the primary to end up greater than or similar to 0.6 M-circle dot greater than the secondary NS, in contradiction with the very nearly equal masses measured in binary neutron stars to date. We point out that the primary NS will evolve into a low-mass black hole (LMBH) as a consequence of hypercritical accretion. Using a flat distribution in the mass ratio q (M-equivalent to(secondary)/M-primary), favored by Duquennoy & Mayor, we calculate the ratio of LMBH- NS to NS-NS systems to be similar to 5, in rough agreement with the result of Pinsonneault & Stanek. The latter authors emphasized the importance of "twins,'' which we discuss. The two neutron stars in twin systems would be close in mass and further increase the number of mergers.