The masses of the millisecond pulsar J1012+5307 and its white dwarf companion

We report on spectroscopy of the white dwarf companion of the millisecond radio pulsar PSR J1012+5307. We find strong Balmer absorption lines, as would be expected for a cool DA white dwarf. The profiles are much narrower than usual, however, and lines are seen up to H12, indicating that the companion has a low gravity and hence a low mass. This is consistent with the expectation-based on evolutionary considerations and on the mass function-that it is a low-mass white dwarf with a helium core. By comparing the spectra to model atmospheres, we derive an effective temperature T-eff=8550+/-25 K and a surface gravity logg=6.75+/-0.07 cgs. Using the Hamada-Salpeter mass-radius relation for helium white dwarfs, with an approximate correction for finite-temperature effects, we infer a mass M(WD)=0.16+/-0.02 M.. This is the lowest mass among all spectroscopically identified white dwarfs. We determine radial velocities from our spectra, and we find a radial-velocity amplitude of 280+/-15 km s(-1). With the pulsar's radial-velocity amplitude, the mass ratio M(PSR)/M(WD)=13.3+/-0.7. From all constraints, we find that with 95% confidence, 1.5 < M(PSR)/M. <3.2.