doi.eso.org

• Bodensteiner, Julia [European Southern Observatory (ESO)]
• Heida, Marianne [European Southern Observatory (ESO)]
• Abdul-Masih, Michael [European Southern Observatory (ESO)]
• Baade, Dietrich [European Southern Observatory (ESO)]
• Banyard, Gareth [Institute of Astronomy, KU Leuven, Belgium]
• Bowman, Dominic M. [Institute of Astronomy, KU Leuven, Belgium]
• Fabry, Matthias [Institute of Astronomy, KU Leuven, Belgium]
• Frost, Abigail [Institute of Astronomy, KU Leuven, Belgium]
• Mahy, Laurent [Royal Observatory of Belgium, Brussels, Belgium]
• Marchant, Pablo [Institute of Astronomy, KU Leuven, Belgium]
• Mérand, Antoine [European Southern Observatory (ESO)]
• Reggiani, Maddalena [Institute of Astronomy, KU Leuven, Belgium]
• Rivinius, Thomas [European Southern Observatory (ESO)]
• Sana, Hugues [Institute of Astronomy, KU Leuven, Belgium]
• Selman, Fernando [European Southern Observatory (ESO)]
• Shenar, Tomer [Anton Pannekoek Institute for Astronomy, University of Amsterdam, the Netherlands]

While the number of stellar-mass black holes detected in X-rays or as gravitational wave sources is steadily increasing, the known population remains orders of magnitude smaller than predicted by stellar evolution theory. A significant fraction of stellar-mass black holes is expected to hide in X-ray-quiet binaries where they are paired with a “normal” star. Although a handful of such quiescent black hole candidates have been proposed, the majority have been challenged by follow-up investigations. A confusion that emerged recently concerns binary systems that appear to contain a normal B-type star with an unseen companion, believed to be a black hole. On closer inspection, some of these seemingly normal B-type stars instead turn out to be stars stripped of most of their mass through an interaction with their binary companion, which in at least two cases is a rapidly rotating star rather than a compact object. These contaminants in the search for quiescent black holes are themselves extremely interesting objects as they represent a rare phase of binary evolution, and should be given special attention when searching for binaries hosting black holes in large spectroscopic studies.