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Astron. Astrophys. 352, 619-622 (1999)

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1. Introduction

If the strength of magnetic field on a white dwarf surface is [FORMULA] [FORMULA]G, the continuum radiation can be noticeably polarized. Such polarization can be detected and studied with conventional broad-band polarimetry. If the magnetic and rotational axes do not coincide (oblique rotator), stellar rotation gives rise to variations in the broad-band polarization. About one-fourth of magnetic white dwarfs have been observed to be polarimetric variables and about half of known objects have been intensively studied to search for the variability (Landstreet, 1992). Most of the variability periods range from several hours to several days. An unusually short period has been found recently in RE J0317-853 (725 sec - Barstow et al. 1995), while KUV 2316+123 has the longest known period - 17.86 days.

There is a group consisting of five magnetic white dwarfs, Grw+70 o8247, LP 790-29, G 240-72, G 227-35 and GD 229, in which no apparent polarization variations have been found on time scales from tens of minutes to more than ten years (West, 1989, Schmidt & Norsworthy, 1990). All these objects have a surface field strength of about a few hundred MegaGauss (MG) and show a large degree of polarization. The absence of variability can be explained either by co-alignment of the magnetic and rotational axes (symmetric rotator) or by a very long ([FORMULA] 100 years) rotational period. The latter explanation could infer the `magnetic braking' mechanism, resulting from magnetically driven transfer of the angular momentum of the progenitor star to a stellar wind during the giant stage and to an expanding envelope during the following stage of collapse. Theoretical considerations show that such braking may be very effective, slowing down the rotational velocity to zero (Pacini, 1970; Hardorp, 1974; Brecher & Chanmugam, 1978) However, given merely the fact of non-variability, it is not easy to choose between these two possibilities. On the other hand, detection of long-term variations over decades would support the hypotesis of very slow rotation.

The first polarization measurements for most of these objects have been made in the 1970s, while the latest observations which did not reveal variability were made in 1986-88 by West (1989). We present new polarization measurements of two white dwarfs belonging to the group of apparent non-variables: GD 229 and G 240-72.

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999