Fig. 2a-e. An example evolutionary path leading to formation of a neutron star neutron star binary: a  initial binary; b  after tidal circularization when both stars expanded to giant sizes, system ejects common envelope; c  system after common envelope phase, star 1 explodes as supernova; d  system after first supernova, now star 2 explodes as supernova; e  system after second supernova: neutron star neutron star binary.