RE J0317 - 853: the hottest known highly magnetic DA white dwarf

We report the discovery of a new highly magnetic DA white dwarf, RE J0317 – 853, through its detection as an EUV source by the ROSAT Wide Field Camera. A combination of far-UV and optical spectra indicates that the star has an effective temperature ≈ 50 000 K, making it the hottest known white dwarf with a measurable magnetic field. Furthermore, the magnetic field is exceptionally intense, having a value ≈ 340 MG, sufficient to shift and broaden the Balmer line features almost beyond recognition. A field of this value is also among the highest known. While these parameters already make this star unique, we have also found that, alone among the magnetic DA white dwarfs, it is a large-amplitude ( ≈ ± 0.1 mag) oscillator at optical wavelengths with a period of 725.4 s. Since there is no known pulsational instability strip in this temperature range, the most plausible explanation is that the modulation of the optical flux is due to rotation of the star. If the star is at the same distance as another DA star, some 16 arcsec away, it must have an unusually small radius and, consequently, a mass approaching the Chandrasekhar limit. The two stars would also comprise one of only three known visual double degenerate systems with hot white dwarf components. ROSAT data indicate that heavy elements detected in the IUE spectrum are mainly attributable to circumstellar material, and we suggest this may arise from a wind driven by cyclotron radiation pressure from the intense magnetic field.