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 A reveal a strongly structured circular polarization spectrum with a peak polarization of 8 percent. The strongest features in the polarization spectrum are attributed to H-alpha 2s0-3p1, 2s0-3p0, and H-beta 2s0-4f-1, 2s0-4f0, and 2s0-4p1 in a magnetic field of 100-300 MG. The phase-averaged data are modelled using centered and off-centered dipole field structures, and a reasonable fit to the data was obtained for a dipole of strength Bd = 450 MG displaced from the center of the star by 35 percent of the stellar radius and viewed at a mean angle of 30-60 deg to the dipole axis. The wavelength-averaged circular polarization data show variations at a period of 725 +/- 10 s. 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.