EUVE J0317 - 855: A rapidly rotating, high-field magnetic white dwarf

Spectropolarimetric observations of the extreme-ultraviolet (EUV) source and high-field magnetic white dwarf EUVE J0317 - 855 (= RE J0317 - 853) in the wavelength range 4200-7200 Å reveal a strongly structured circular polarization spectrum with a peak polarization of 8 per cent. The strongest features in the polarization spectrum are attributed to Hα 2s0-3p1, 2s0-3p0 and Hβ 2s0-4f-1, 2s0-4f0 and 2s0-4p1 in a magnetic field of 100-300 MG. The phase-averaged data are modelled using centred and off-centred dipole field structures, and a reasonable fit to the data was obtained for a dipole of strength B_d=450 MG displaced from the centre of the star by 35 per cent of the stellar radius and viewed at a mean angle of 30°-60° to the dipole axis. The wavelength-averaged circular polarization data show variations at a period of 725 ± 10 s, consistent with the photometric period of 725 s reported by Barstow et al. We also report EUV photometric variations on a period of 725.5 ± 0.8 s. The EUV light curve is double-peaked, a possible signature of surface abundance inhomogeneities, in contrast with the optical light curve which is single-peaked on the same period. We show that the optical photometric and polarization variations are most likely caused by changes in the surface-averaged field strength and the line-of-sight projected field structure with rotational phase, as would be expected from an oblique rotator model. The high rotation rate, high mass and age discrepancy with a close DA white dwarf companion (LB 9802) suggest that EUVE J0317 - 855 may be the outcome of a double-degenerate merger.