EUVE J0317-855 - A RAPIDLY ROTATING, HIGH-FIELD MAGNETIC WHITE-DWARF

Spectropolarimetric observations of the extreme-ultraviolet (EUV) sour ce and high-field magnetic white dwarf EUVE J0317 - 855 (=RE J0317 - 8 53) in the wavelength range 4200-7200 Angstrom reveal a strongly struc tured circular polarization spectrum with a peak polarization of 8 per cent. The strongest features in the polarization spectrum are attribu ted to H alpha 2s0-3p1, 2s0-3p0 and H beta 2s0-4f-1, 2s0-4f0 and 2s0-4 p1 in a magnetic field of 100-300 MG. The phase-averaged data are mode lled using centred and off-centred dipole held structures, and a reaso nable 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 degrees-60 degrees to the dip ole axis. The wavelength-averaged circular polarization data show vari ations at a period of 725 +/- 10 s, consistent with the photometric pe riod of 725 s reported by Barstow et al. We also report EUV photometri c variations on a period of 725.5 +/- 0.8 s. The EUV light curve is do uble-peaked, a possible signature of surface abundance inhomogeneities , in contrast with the optical light curve which is single-peaked on t he same period. We show that the optical photometric and polarization variations are most likely caused by changes in the surface-averaged f ield strength and the line-of-sight projected field structure with rot ational phase, as would be expected from an oblique rotator model. The high rotation rate, high mass and age discrepancy with a close DA whi te dwarf companion (LB 9802) suggest that EUVE J0317 - 855 may be the outcome of a double-degenerate merger.