Optical linear polarization of helium-rich white dwarfs samplewith the RoboPol polarimeter.

We performed observations of 28 white dwarfs (WDs) as the second part of our WD optical linear polarimetric survey. Our study shows that highly linear polarized WDs are very rare. However, three systems, WD1336+123 (DB+dM4), WD1415+234 (DBA) and WD1619+123 (DA+dG0), show linear polarization degree higher than 1 per cent. In particular, WD1415+234 with polarization degree ∼2.6 per cent is the strongest polarized WD in our total sample of 101 objects. We found that the median polarization degree of isolated DA WDs and DB WDs is similar but lower than the median polarization of isolated DC WDs. The DB WDs in binaries seem to be more polarized than binaries containing DA+dM or double-degenerated systems (DDSs) with DA component. Additionally, we present for the first time, the measurements of optical linear polarization of both components in two common proper motion binary systems (CPMB) systems: WD1336+123 and the Sirius-like system WD2129+000. Because the vast majority of WDs have very low polarization degree (<0.5 per cent), they can be used as faint linear polarization standards. Our sample covers SDSS r magnitude range from 13 to 17 and right ascension range from 13^h to 24^h, with the extension from 1^h to 3^h. [ABSTRACT FROM AUTHOR]