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 \sim 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 \pm 6 AU projected separation to the B9-type stellar pair .
With a model-dependent mass of 16-25 M _ { Jup } 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 .