  Little is known about the incidence of magnetic fields among the coolest
white dwarfs. Their spectra usually do not exhibit any absorption lines as the
bound-bound opacities of hydrogen and helium are vanishingly small. Probing
these stars for the presence of magnetic fields is therefore extremely
challenging. However, external pollution of a cool white dwarf by, e.g.,
planetary debris, leads to the appearance of metal lines in its spectral energy
distribution. These lines provide a unique tool to identify and measure
magnetism in the coolest and oldest white dwarfs in the Galaxy.
  We report the identification of 7 strongly metal polluted, cool (T_eff < 8000
K) white dwarfs with magnetic field strengths ranging from 1.9 to 9.6 MG. An
analysis of our larger magnitude-limited sample of cool DZ yields a lower limit
on the magnetic incidence of 13+/-4 percent, noticeably much higher than among
hot DA white dwarfs.
