Abstract
In this article we study the nature of the recently identified populations of hot companions to red supergiant stars (RSGs). To this end, we compile the literature on the most well characterised systems with the aim of better understanding the hot companions identified with ultra-violet photometry and confirmed with Hubble Space Telescope spectra in the Local Group. We identify nine systems with current masses greater than around 8M⊙ that have constraints on their orbital periods, which are in the range of 3 to 75 yr. Antares (α Sco) is the obvious outlier in this distribution, having an estimated orbital period of around 2000 yr. Mass-ratio (q = M2/MRSG) estimates are available only for five of the compiled systems and range between 0.16 and 1. Ongoing efforts to identify and characterise hot companions to RSGs in the SMC have revealed 88 hot companions that have observational constraints free of contamination from the RSG component and show a flat-q distribution in the range 0.3 < q < 1. We present a summary of a recently conducted HST UV spectroscopic survey that aims to characterise a subset of these companions.
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