1999A&A...352..619B

GD 229 and G 240-72 belong to the group of strongly polarized magnetic (B≥10⁸) 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 ≥ 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.