Search for indications of fast rotation in the linear polarization of the magnetic white dwarf Grw+70°8247

Grw+70 degrees 8247 belongs to a small group of magnetic white dwarfs which despite repeated observations did not show any changes in their polarization over decades. The inferred rotation periods are of the order of hundred years which is not easy to understand if angular momentum is: conserved during stellar evolution. An alternative possibility, which was to our knowledge never investigated, might be fast rotation with a period between some minutes and a couple of seconds. Assuming a mass of 1 M-. and a radius of 1/100 R-. the shortest possible period is about 10 s. In order to test this hypothesis the linear polarization of Graw+70 degrees 8247 was measured at the 2.2 m telescope at the Calar Alto Observatory, Spain, with a rime resolution of 3 s by trailing the linear polarized image of Grw+70 degrees 8247 over the CCD chip. The linear polarization was searched for periodicities in the range between 10 s: and 4000 s. Investigations with synthetic polarized data using the same window function and an assumed sinusoidal variation showed that variations with amplitudes of 40% relative to the amount of average linear polarization should be detectable. However, no periodicities at this or higher levels of significance were found. This would be in accordance with a recent report on slow rotation of GD 229 and G 240-72, but fast variations with lower amplitudes could not be ruled out.