The mass of the neutron star in the binary millisecond pulsar PSR J1012 + 5307

We have measured the radial velocity variation of the white dwarf secondary in the binary system containing the millisecond pulsar PSR J1012 + 5307. Combined with the orbital parameters of the radio pulsar, we infer a mass ratio q (≡M₁/M₂) = 10.5 ± 0.5. Our optical spectroscopy has also allowed us to determine the mass of the white dwarf companion by fitting the spectrum to a grid of DA model atmospheres: we estimate M₂ = 0.16 ± 0.02M_⊙, and hence the mass of the neutron star is 1.64 ± 0.22 M_⊙, where the error is dominated by that of M₂. The orbital inclination is 52 ± 4 deg. For an initial neutron star mass of ∼1.4 M_⊙, only a few tenths of a solar mass at most has been successfully accreted over the lifetime of the progenitor low-mass X-ray binary. If the initial mass of the secondary was ∼1 M_⊙, our result suggests that the mass transfer may have been non-conservative.