5489 20230123212203.0 10.1051/0004-6361/202141037 DOI SCART-2021-0149 Banyard, G. The observed multiplicity properties of B-type stars in the Galactic young open cluster NGC 6231 2022 It is well known that massive O-stars are frequently (if not always) found in binary or higher-order multiple systems, but this fact has been less robustly investigated for the lower mass range of the massive stars, represented by the B-type stars. Obtaining the binary fraction and orbital parameter distributions of B-type stars is crucial to understand the impact of multiplicity on the archetypal progenitor of core-collapse supernovae as well as to properly investigate formation channels for gravitational wave progenitors. This work aims to characterise the multiplicity of the B-star population of the young open cluster NGC 6231 through multi-epoch optical spectroscopy of 80 B-type stars. We analyse 31 FLAMES/GIRAFFE observations of 80 B-type stars, monitoring their radial velocities (RVs) and performing a least-squares spectral analysis (Lomb-Scargle) to search for periodicity in those stars with statistically significant variability in their RVs. We constrain an observed spectroscopic binary fraction of $33\pm5$% for the B-type stars of NGC 6231, with a first order bias-correction giving a true spectroscopic binary fraction of $52\pm8$%. Out of 27 B-type binary candidates, we obtained orbital solutions for 20 systems: 15 single-lined (SB1) and 5 double-lined spectroscopic binaries (SB2s). We present these orbital solutions and the orbital parameter distributions associated with them. Our results indicate that Galactic B-type stars are less frequently found in binary systems than their more massive O-type counterparts, but their orbital properties generally resemble those of B- and O-type stars in both the Galaxy and Large Magellanic Cloud. NO binaries: spectroscopic stars: early-type stars: massive open clusters and associations: individual: NGC 6231 Astrophysics - Solar and Stellar Astrophysics Astrophysics - Astrophysics of Galaxies Sana, H. Mahy, L. Bodensteiner, J. Villaseñor, J. I. Evans, C. J. Astronomy & Astrophysics 658 2022 A69 28 laurent.mahy@ksb-orb.be 6053206 http://publi2-as.oma.be/record/5489/files/Banyard_2022_aa41037-21.pdf 3427 http://publi2-as.oma.be/record/5489/files/Banyard_2022_aa41037-21.gif?subformat=icon icon 7457 http://publi2-as.oma.be/record/5489/files/Banyard_2022_aa41037-21.jpg?subformat=icon-180 icon-180 published in REFERD