  About 20% of the white dwarfs possess a magnetic field that may be detected
by the splitting and/or polarization of their spectral lines. As they cool, the
effective temperatures of the white dwarfs becomes so low that no spectral
lines can be seen in the visible wavelength range. If their atmospheres are not
polluted by the debris of a planetary system, these cool white dwarfs have
featureless optical spectra. Until quite recently, very little was known about
the incidence of magnetic fields in these objects. However, when observed with
polarimetric techniques, a significant number of featureless white dwarfs
reveal strong magnetic fields in their optical continuum spectra. Measuring the
occurrence rate and strength of magnetic fields in old white dwarfs may help us
to understand how these fields are generated and evolve. We report the results
of an ongoing survey of cool white dwarfs with the high-precision broad-band
polarimeter DIPOL-UF, which is deployed at the Nordic Optical Telescope on La
Palma, Spain. This survey has led to the firm discovery of 13 cool magnetic
white dwarfs in the solar neighborhood so far, including six new detections
that we report in this paper.
