In the latest stages of white dwarf evolution, when they are so cool that spectral lines are no longer formed in hydrogen- or helium-dominated atmospheres, magnetic fields can be probed only by means of circular polarimetry of the continuum. The study of the fields of white dwarfs without spectral features (DC class) may reveal whether Ohmic decay acts on magnetic white dwarfs, or if magnetic fields continue to be generated even several billion years after the white dwarf formation. Broad-band circular polarisation measurements with the DIPol-UF polarimeter, developed by KIS in cooperation with the University of Turku, Finland, have the advantage of reaching a higher accuracy in the continuum, with the potential of detecting magnetic fields as weak as a fraction of a Mega-Gauss in such white dwars. The first observing campaign with the DIPol-UF polarimeter have led to the discovery of two new magnetic white dwarfs of spectral class DC, probably the first discovery of this kind made with broad-band circular polarimetric techniques since the late 1970s.
Reference: Berdyugin, A., Piirola, V., Bagnulo, S., Landstreet, J.D., Berdyugina, S.V. 2021, Highly sensitive search for magnetic fields in white dwarfs using broad-band circular polarimetry, A&A, in press, arxiv:2111.11174