  Wide low-mass substellar companions are known to be very rare among low-mass
stars, but appear to become increasingly common with increasing stellar mass.
However, B-type stars, which are the most massive stars within ~150 pc of the
Sun, have not yet been examined to the same extent as AFGKM-type stars in that
regard. In order to address this issue, we launched the ongoing B-star
Exoplanet Abundance Study (BEAST) to examine the frequency and properties of
planets, brown dwarfs, and disks around B-type stars in the Scorpius-Centaurus
(Sco-Cen) association; we also analyzed archival data of B-type stars in
Sco-Cen. During this process, we identified a candidate substellar companion to
the B9-type spectroscopic binary HIP 79098 AB, which we refer to as HIP 79098
(AB)b. The candidate had been previously reported in the literature, but was
classified as a background contaminant on the basis of its peculiar colors.
Here we demonstrate that the colors of HIP 79098 (AB)b are consistent with
several recently discovered young and low-mass brown dwarfs, including other
companions to stars in Sco-Cen. Furthermore, we show unambiguous common proper
motion over a 15-year baseline, robustly identifying HIP 79098 (AB)b as a bona
fide substellar circumbinary companion at a 345+/-6 AU projected separation to
the B9-type stellar pair. With a model-dependent mass of 16-25 Mjup yielding a
mass ratio of <1%, HIP 79098 (AB)b joins a growing number of substellar
companions with planet-like mass ratios around massive stars. Our observations
underline the importance of common proper motion analysis in the identification
of physical companionship, and imply that additional companions could
potentially remain hidden in the archives of purely photometric surveys.
