The cool magnetic DAZ white dwarf NLTT 10480

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 '' yr(-1)) shows weak Zeeman-split lines of calcium as well as characteristic H alpha and beta Zeeman triplets. Using VLT X-shooter spectra, we measured a surface-averaged magnetic field B(S) similar to 0.5 MG. The relative intensity of the pi and sigma components of the calcium and hydrogen lines imply a high inclination (i less than or similar to 60 degrees). The optical-to-infrared V - J colour index and the Ca I/Ca II 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 less than or similar to 10(-4) solar. Assuming diffusion steady-state, a calcium accretion rate of log. (M) over dot (g s(-1)) = 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.