  We have identified a new, cool magnetic white dwarf in the New Luyten
Two-Tenths (NLTT) catalogue. The high proper-motion star NLTT 10480
(mu=0.5"/year) shows weak Zeeman-split lines of calcium, as well as
characteristic Halpha and beta Zeeman triplets. Using VLT X-shooter spectra we
measured a surface-averaged magnetic field B_S ~ 0.5 MG. The relative intensity
of the pi and sigma components of the calcium and hydrogen lines imply a high
inclination (i > 60 deg). The optical-to-infrared V-J colour index and the
CaI/CaII ionization balance indicate a temperature between 4900 and 5200 K,
while the Balmer line profiles favour a higher temperature of 5400 K. The
discrepancy is potentially resolved by increasing the metallicity to 0.03x
solar, hence increasing the electron pressure. However, the measured calcium
abundance and abundance upper limits for other elements (Na, Al, Si. and Fe)
imply a low photospheric metallicity < 10^{-4} solar. Assuming diffusion
steady-state, a calcium accretion rate of log(dM/dt)(g/s)=5.6+/-0.3 is required
to sustain a calcium abundance of log(n(Ca)/n(H))=-10.30+/-0.30 in the white
dwarf atmosphere. We examine the implications of this discovery for the
incidence of planetary debris and weak magnetic fields in cool white dwarf
stars.
