By means of fitting a linear trend together with a sinusoidal oscillation a new and more accurate period of h was derived for KPD 0253+5052 . This period is significantly (6 ) shorter than the older value of 4.1 h derived by Schmidt & Norsworthy (1991 ) and the error is reduced by a factor of 10.
Rotating magnetic white dwarf stars offer the possibility to determine all parameters describing the magnetic field geometry, like inclination of the magnetic field axis, field strength and deviations from the dipole geometry. As all spectra are modulated according to the rotation period, a erroneous rotation period will result in wrong phase shifts - in this case a whole period in about two days.
Since the investigation of period distributions among magnetic white dwarfs of Schmidt & Norsworthy (1991 ) hardly any progress has been made. At that time nine magnetic white dwarfs were known spinning with periods up to 18 days and five being polarimetrically constant. The latter were given periods longer than 100 years since all show highly polarized continua and variations should be easily detectable. From the observation of main sequence stars together with conservation of angular momentum much shorter rotation periods for white dwarfs would be expected. Transferring magnetically angular momentum to the interstellar medium could be an explanation for the slow rotation of white dwarfs. Thus a relationship between the magnetic field strength and the period should exist in the sense that a stronger field corresponds to a longer period.
Meanwhile some new stars can be added to the sample. Two of them, J0317-853 (Barstow et al. 1995 ) and 1211-1707 (Reimers et al. 1996 ) are spinning with periods lower than 100 minutes. In addition J0317-853, with 725s the fastest rotating magnetic white dwarf known so far, has a dipole field strength of 340 MG , and 1211-1707 one of 80 MG , suggesting that there is no correlation between period and magnetic field strength.
As more and more spectropolarimetric data become available there seems to be some indication that deviations from a pure dipole magnetic field geometry are common among magnetic white dwarfs. About a quarter of all magnetic white dwarfs show multipole or offcentered dipole field geometries. According to Putney & Jordan (1995 ) this is in agreement with the magnetic field geometries found for Ap stars under the assumption that the field structure is conserved during stellar evolution. There are also a few magnetic Ap stars with very long periods of hundreds of days and more (Catalano & Renson 1984 ). Together with a similar space density of Ap and magnetic white dwarf stars (e.g. Angel et al. 1981 ) this is speaking in favor of Ap stars being the progenitors of magnetic white dwarfs.
© European Southern Observatory (ESO) 1997
Online publication: April 20, 1998