see Sect. \[sec3-5\]). Comparing this to the histograms in Figure 4 of shows how S2 0109+22 varies more rapidly in polarization angle than the average TeV (mean $1.11\pm0.29$ deg./day) and non-TeV (mean $1.66\pm0.45$ deg./day) sources. This is also seen when we examine the polarization variations in the $Q/I-U/I$-plane (see the inset in Fig. \[pollc\] for the $Q/I-U/I$ plot). As described in a tightly clustered distribution of the points in the $Q/I-U/I$-plane is an indication of a preferred polarization angle. For S2 0109+22 the weighted average of the $Q/I$ and $U/I$ values places the mass center at a distance of 0.039 from the origin, which is smaller than the mean value of $0.050\pm0.008$ for the TeV sources in . However, the spread in the points, quantified as the distance of each point from the mass center, is 0.077, which is much higher than the mean values ($0.021\pm0.003$ for TeV and $0.041\pm0.005$ for non-TeV sources) in . In fact, there is only one non-TeV source with a value higher than we obtain for S2 0109+22.

These results are in good agreement with previous studies and indicate that the optical polarization of S2 0109+22 is more variable both in fractional polarization and position 