Six of the eight double neutron stars known in the Galactic disk have low orbital eccentricities ( < 0.27 ) indicating that their second-born neutron stars received only very small velocity kicks at birth . This is similar to the case of the B-emission X-ray binaries , where a sizable fraction of the neutron stars received hardly any velocity kick at birth ( Pfahl et al . 2002 ) . The masses of the second-born neutron stars in five of the six low-eccentricity double neutron stars are remarkably low ( between 1.18 and 1.30M _ { \odot } ) . It is argued that these low-mass , low-kick neutron stars were formed by the electron-capture collapse of the degenerate O-Ne-Mg cores of helium stars less massive than about 3.5M _ { \odot } , whereas the higher-mass , higher kick-velocity neutron stars were formed by the collapses of the iron cores of higher initial mass . The absence of low-velocity single young radio pulsars ( Hobbs et al . 2005 ) is consistent with the model proposed by Podsiadlowski et al . ( 2004 ) , in which the electron-capture collapse of degenerate O-Ne-Mg cores can only occur in binary systems , and not in single stars .