  We present the discovery and multi-wavelength characterisation of VVV
J1438-6158 AB, a new field wide-binary system consisting of a 4.6(+5.5-2.4) Gyr
and Teff = 9500+/-125 K DA white dwarf (WD) and a Teff = 2400+/-50 K M8
ultracool dwarf (UCD). The projected separation of the system is a = 1236.73 au
(~13.8"), and although along the line-of-sight towards the Scorpius-Centaurus
(Sco-Cen) stellar association, VVV J1438-6158 AB is likely to be a field star,
from a kinematic 6D probabilistic analysis. We estimated the physical, and
dynamical parameters of both components via interpolations with theoretical
models and evolutionary tracks, which allowed us to retrieve a mass of
0.62+/-0.18 MSun for the WD, and a mass of 98.5+/-6.2 MJup (~0.094+/-0.006
MSun) for the UCD. The radii of the two components were also estimated at
0.01309+/-0.0003 RSun and 1.22+/-0.05 RJup, respectively. VVV J1438-6158 AB
stands out as a benchmark system for comprehending the evolution of WDs and
low-mass companions given its status as one of the most widely separated WD+UCD
systems known to date, which likely indicates that both components may have
evolved independently of each other, and also being characterised by a large
mass-ratio (q = 0.15+/-0.04), which likely indicates a formation pathway
similar to that of stellar binary systems.
