  Our goal is to characterise the physical properties of the metal-poor brown
dwarf population. In particular, we focus on the recently discovered peculiar
dwarf WISE J1810055$-$1010023.
  We collected optical iz and near-infrared J-band imaging on multiple
occasions over 1.5 years to derive accurate trigonometric parallax and proper
motion of the metal-depleted ultra-cool dwarf candidate WISE1810. We also
acquired low-resolution optical spectroscopy (0.6$-$1.0 $\mu$m) and new
infrared (0.9$-$1.3 $\mu$m) spectra of WISE1810 that were combined with our
photometry, other existing data from the literature and our trigonometric
distance to determine the object's luminosity from the integration of the
observed spectral energy distribution covering from 0.6 through 16$\mu$m. We
compared the full optical and infrared spectrum with state-of-the-art
atmosphere models to further constrain its effective temperature, surface
gravity and metallicity.
  WISE1810 is detected in the $iz$ bands with AB magnitudes of
$i$=23.871$\pm$0.104 and $z$=20.147$\pm$0.083 mag in the PanSTARRS system. It
does not show any obvious photometric variability beyond 0.1$-$0.2 mag in any
of the $z$- and $J$-band filters. The very red $z-J \approx 2.9$ mag colour is
compatible with an ultra-cool dwarf nature. Fitting for parallax and proper
motion, we measure a trigonometric parallax of 112.5$^{+8.1}_{-8.0}$ mas for
WISE1810, placing the object at only 8.9$^{+0.7}_{-0.6}$ pc, about three times
closer than previously thought. We employed Monte Carlo methods to estimate the
error on the parallax and proper motion. The object's luminosity was determined
at log$L/L_\odot$=$-$5.78$\pm$0.11 dex. From the comparison to atmospheric
models, we infer a likely metallicity of [Fe/H] $\approx -1.5$ and an effective
temperature cooler than 1000K.
  Abridged
