WD 1953-011: A magnetic white dwarf with peculiar field structure

We present Hα spectra of the magnetic white dwarf star WD 1953-011 which confirm the presence of the broad Zeeman components corresponding to a field strength of ∼500 kG found by Maxted & Marsh. We also find that the line profile is variable over a time-scale of a day or less. The core of the Hα line also shows a narrow Zeeman triplet corresponding to a field strength of ∼100 kG which appears to be almost constant in shape. These observations suggest that the magnetic field on WD 1953-011 has a complex structure, and that the star has a rotational period of hours or days which causes the observed variability of the spectra. We argue that neither an offset dipole model nor a double-dipole model is sufficient to explain our observations. Instead, we propose a two-component model consisting of a highfield region of magnetic field strength ∼500 kG covering about 10 per cent of the surface area of the star, superimposed on an underlying dipolar field of mean field strength ∼70 kG. Radial velocity measurements of the narrow Zeeman triplet show that the radial velocity is constant to within a few kms^-1, so this star is unlikely to be a close binary.