Unique White Dwarfs Accompanying Recycled Pulsars

Knowledge of the properties of the white-dwarf companions of radio pulsars can provide unique constraints on the characteristics and evolution of these binaries, as well as on those of its constituents. Many white-dwarf properties can be determined accurately from their spectra, but for the very faint pulsar companions spectroscopy has only become feasible with the advent of large telescopes like the Keck. Here, I introduce the two classes of pulsar, white-dwarf binaries, and describe for each what we have learned from a specific system, PSR J1012+5307 and PSR B0655+64, respectively, summarising what has been done (Van Kerkwijk & Kulkarni 1995; Van Kerkwijk, Bergeron, & Kulkarni 1996; hereafter Papers I & II), presenting new results, and discussing what the future may hold. Briefly, for the companion of PSR J1012+5307 we find a DA spectrum, and infer a mass of ∼0.16 M⊙, the lowest among all spectroscopically identified white dwarfs. Combined with a radial-velocity orbit, a neutron-star mass between 1.5 and 3.2 M⊙ (95% confidence) is derived. The companion of PSR B0655+64 shows strong Swan C2 bands, i.e., it is a DQ star. Unlike anything reported for other DQs, however, it shows variations in strength of the bands by a factor two. Most likely, the variations are periodic, with a period of ∼ 9.7 h. This is substantially shorter than the 1 day orbital period, which can likely be understood in terms of its past evolution.