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

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4. Discussion

We can say now that polarization of GD 229 is definetely variable on the time scale of about 10 years. In the B band, over the last 20 years, the degree of polarization has increased from 2.8% to 4.0%, while the position angle has rotated from 80o to 114o (see Fig. 2). If the observed polarization variations are due to the slow rotation, the rotational period of GD 229 can be about 80-100 years. Repeating observations in the next 10-15 years will allow us to make a more accurate estimation.

[FIGURE] Fig. 2. Variations of the linear polarization and position angle of GD 229 in B band from December 1973 to November 1996. Vertical bars are 2[FORMULA] errors.

We can also conclude that there is an evidence of long-term polarization variations in G 240-72. Such variations, however, are seen only on time-scales of more than 20 years. This suggests a rotational period which is at least two times longer than in GD 229. Along with the magnetic field strength, the total amount of mass loss is another important factor determining braking efficiency (e.g. Pacini, 1970). As the surface field strength in both white dwarfs is of [FORMULA]G (well above the [FORMULA]G limit required for effective braking), the slower rotation of G 240-72 is probably resulted from smaller mass loss on the stage of formation in comparison with GD 229.

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999