2010MNRAS.404..159G

We confirm SDSSJ130033.48+590407.0 as a common proper motion companion to the well-studied hydrogen-atmosphere (DA) white dwarf PG1258+593 (GD322). The system lies at a distance of 68±3pc, where the angular separation of 16.1±0.1arcsec corresponds to a minimum binary separation of 1091±7au. SDSSJ1300+5904 is a cool (T_eff= 6300±300K) magnetic white dwarf (B ≃ 6mG). PG1258+593 is a DA white dwarf with T_eff= 14790 ±77K and logg = 7.87±0.02. Using the white dwarf mass-radius relation implies the masses of SDSSJ1300+5904 and PG1258+593 are 0.54±0.06 and 0.54 ±0.01M_☉, respectively, and therefore a cooling age difference of 1.67 ±0.05Gyr. Adopting main-sequence lifetimes from stellar models, we derive an upper limit of 2.2M_☉ for the mass of the progenitor of PG1258+593. A plausible range of initial masses is 1.4-1.8 M_☉ for PG1258+593 and 2-3 M_☉ for SDSSJ1300+5904. Our analysis shows that white dwarf common proper motion binaries can potentially constrain the white dwarf initial mass-final mass relation and the formation mechanism for magnetic white dwarfs. The magnetic field of SDSSJ1300+5904 is consistent with an Ap progenitor star. A common envelope origin of the system cannot be excluded, but requires a triple system as progenitor.