  White dwarfs stars are known to be polluted by their active planetary
systems, but little attention has been paid to the accretion of wind from
low-mass companions. The capture of stellar or substellar wind by white dwarfs
is one of few methods available to astronomers which can assess mass-loss rates
from unevolved stars and brown dwarfs, and the only known method to extract
their chemical compositions. In this work, four white dwarfs with
closely-orbiting, L-type brown dwarf companions are studied to place limits on
the accretion of a substellar wind, with one case of a detection, and at an
extremely non-solar abundance $m_{\rm Na}/m_{\rm Ca}>900$. The mass-loss rates
and upper limits are tied to accretion in the white dwarfs, based on limiting
cases for how the wind is captured, and compared with known cases of wind
pollution from close M dwarf companions, which manifest in solar proportions
between all elements detected. For wind captured in a Bondi-Hoyle flow,
mass-loss limits $\dot M\lesssim 5\times10^{-17}$ M$_\odot$ yr$^{-1}$ are
established for three L dwarfs, while for M dwarfs polluting their hosts, winds
in the range $10^{-13} - 10^{-16}$ M$_\odot$ yr$^{-1}$ are found. The latter
compares well with the $\dot M\sim 10^{-13} - 10^{-15}$ M$_\odot$ yr$^{-1}$
estimates obtained for nearby, isolated M dwarfs using Ly$\alpha$ to probe
their astropsheres. These results demonstrate that white dwarfs are
highly-sensitive stellar and substellar wind detectors, where further work on
the actual captured wind flow is needed.
