  Mass-loss in massive stars plays a critical role in their evolution, although
the precise mechanism(s) responsible - radiatively driven winds, impulsive
ejection and/or binary interaction -remain uncertain. In this paper we present
ALMA line and continuum observations of the supergiant B[e] star Wd1-9, a
massive post-Main Sequence object located within the starburst cluster
Westerlund 1. We find it to be one of the brightest stellar point sources in
the sky at millimetre wavelengths, with (serendipitously identified) emission
in the H41alpha radio recombination line. We attribute these properties to a
low velocity (~100 km/s) ionised wind, with an extreme mass-loss rate
6.4x10^-5(d/5kpc)^1.5 Msol/yr. External to this is an extended aspherical
ejection nebula indicative of a prior phase of significant mass-loss. Taken
together, the millimetre properties of Wd1-9 show a remarkable similarity to
those of the highly luminous stellar source MWC349A.We conclude that these
objects are interacting binaries evolving away from the main sequence and
undergoing rapid case-A mass transfer. As such they - and by extension the
wider class of supergiant B[e] stars - may provide a unique window into the
physics of a process that shapes the life-cycle of ~70% of massive stars found
in binary systems.
