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

GD 229 and G 240-72 belong to the group of strongly polarized magnetic (B g reater than or equal to 10(8)) white dwarfs which do not show apparent pola rization variability. New polarimetric observations obtained in November 19 96 and 1997 at the Nordic Optical Telescope clearly confirmed a previously reported detection of long-term variations in polarization of GD 229 and ha ve 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 greater than or e qual to 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.