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 \arcsec 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 } \sim 0.5 MG .
The relative intensity of the \pi and \sigma components of the calcium and hydrogen lines imply a high inclination ( i \ga 60 ^ { \circ } ) .
The optical-to-infrared V - J colour index and the Ca i / Ca ii ionization balance indicate a temperature between 4 900 and 5 200 K , while the Balmer line profiles favour a higher temperature of 5400 K. The discrepancy is potentially resolved by increasing the metallicity to 0.03 \times 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 \la 10 ^ { -4 } solar .
Assuming diffusion steady-state , a calcium accretion rate of \log { \dot { M } ( { g s } ^ { -1 } ) } = 5.6 \pm 0.3 is required to sustain a calcium abundance of \log { n { ( Ca ) } / n { ( H ) } } = -10.30 \pm 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 .