angle than other high-energy BL Lac objects, and that there does not seem to be a preferred polarization angle in the source, at least over our monitoring period. This is not unexpected based on the analysis of which showed that the polarization variability depends more on the position of the synchrotron peak rather than the detection of TeV emission . As shown in Figure \[SED\], in the ISP-type S2 0109+22 the optical emission probes the peak of the synchrotron component, where the variability is expected to be higher [see also @2016MNRAS.463.3365A]. Comparing the obtained intrinsic mean polarization fraction to the values presented by @2016MNRAS.463.3365A, this source seems to be a rather typical ISP-type object. The maximum polarization fraction is over 15%, which is high, but not uncommon for ISP sources, as shown in where about 30% of the ISP objects reach fractional polarization values as high as or higher than 15%. This indicates that the magnetic field order must be fairly high in the emission region.

Redshift estimation {#sec3-5}
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The lack of emission lines in the optical spectrum of BL Lacs objects makes the determination of the redshift of these sources particularly challenging. An estimation on the distance can 