Vol. 556
In section 10. Planets and planetary systems

Astrometric orbit of a low-mass companion to an ultracool dwarf

by J. Sahlmann, P. Lazorenko, D. Segransan, et al., A&A 556, A133

Physical companions to brown dwarfs are usually found in nearly equal-mass configurations and with separations beyond 1 AU, contrasting with the wider mass-ratio and separation distributions of solar-type and M dwarf binaries. That difference, if intrinsic, would represent an important constraint on the formation mechanism of multiple brown dwarfs, but may reflect detection biases. Using astrometric measurements of unprecedented accuracy for ground-based observations of such faint targets, Sahlman et al. detect a companion to DENIS-P J082303.1-491201 with a 246-day period and a 0.36 mass ratio. This opens a new window on low-mass ratio companions at small to intermediate separations around brown dwarfs.