Abstract
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(Na)/m(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 (M) overdot less than or similar to 5 x10(-17) M-circle dot yr(-1) are established for three L dwarfs, while for M dwarfs polluting their hosts, winds in the range 10(-13)-10(-16) M-circle dot yr(-1) are found. The latter compares well with the (M) overdot similar to 10(-13)-10(-15) M-circle dot yr(-1) estimates obtained for nearby, isolated M dwarfs using Lyato 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.