  About half of white dwarfs (WDs) evolve to the DC state as they cool; the
others become DQ or (temporarily?) DZ WDs. The recent magnetic survey of the
local 20 pc volume has established a high frequency of magnetic fields among
WDs older than 2-3 Gyr, demonstrating that in low- and average-mass WDs, the
effects of magnetism become more common as they age, and the fields on average
become stronger. However, the available statistics of WDs older than about 5
Gyr do not clearly establish how fields evolve beyond this age. We are carrying
out a survey to clarify the occurrence of magnetism in DC-type WDs in order to
better understand this late evolution. We use broadband filter polarimetry,
arguably the most efficient way to detect magnetic fields in featureless WDs
via continuum circular polarization. Here we report the discovery of a magnetic
field in five DC WDs (of 23 observed), almost doubling the total sample of
known magnetic WDs belonging to the DC spectral class.
