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

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Polarization variability in magnetic white dwarfs GD 229 and G 240-72

A.V. Berdyugin 1,2,3 and V. Piirola 2

1 Astronomy Division, University of Oulu, Linnanmaa, PL 3000, 90401 Oulu, Finland (andrei@ukko.oulu.fi)
2 Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
3 Crimean Astrophysical Observatory, P.O. Nauchny, Crimea 334413, Ukraine

Received 10 September 1999 / Accepted 30 October 1999

Abstract

GD 229 and G 240-72 belong to the group of strongly polarized magnetic ([FORMULA]) white dwarfs which do not show apparent polarization variability. New polarimetric observations obtained in November 1996 and 1997 at the Nordic Optical Telescope clearly confirmed a previously reported detection of long-term variations in polarization of GD 229 and have given for the first time an evidence of similar variations in G 240-72. As this variability can be seen only on the time scale of [FORMULA] 10 years, the rotational periods of these objects must be about or longer than 100 years. Such an extremely slow rotation is probably a result of magnetic braking.

Key words: stars: magnetic fields – stars: rotation – stars: white dwarfs – stars: individual: GD 229 – stars: individual: G 240-72

Send offprint requests to: A.V.Berdyugin (andrei@ukko.oulu.fi)

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999
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