BLOeM. BAF-type supergiants

As part of The Binarity at LOw Metallicity (BLOeM) campaign in the Small Magellanic Cloud, we present a multi-epoch spectroscopic survey of 128 supergiant stars of spectral type B5-F5, which roughly correspond to initial masses in the 6-30M_{sun}_ range. The observed binary fraction for the B5-9 supergiants is 25+/-6% (10+/-4%) and 5+/-2% (0%) for the A-F stars, which were found using a radial-velocity (RV) variability threshold of 5km/s (10km/s) as a criterion for binarity.

Accounting for observational biases, we find an intrinsic multiplicity fraction of less than 18% for the B5-9 stars and 8^+9^_-7_% for the AF stars, for the orbital periods up to 10^3.5^days and mass ratios (q) in the 0.1<q<1 range. The large stellar radii of these supergiant stars prevent short orbital periods, but we demonstrate that this effect alone cannot explain our results. We assessed the spectra and RV time series of the detected binary systems and find that only a small fraction display convincing solutions.