De zoektocht naar massieve dubbelsterren met een compact object in de Melkweg

Stars born with masses more than eight times the mass of the Sun -- massive stars -- are cosmic engines. They impact and regulate star formation, chemical enrichment, and dynamical feedback in their host galaxies. Some massive stars end their lives in violent supernovae, leaving behind a compact object -- neutron star or black hole. The detection of gravitational waves coming from the merger of two compact objects was one of the greatest astronomical achievements of the last 10 years. The observed black holes that came with the gravitational-wave detections reminded astronomers that we do not understand where these double compact-object mergers originate from. While it is suggested that massive binaries can evolve into gravitational-wave mergers, the exact evolutionary pathway remains unclear. There are two main open questions in this regard: ``How do black holes form?'' and ``What is the role of binary interactions in the evolution towards gravitational-wave mergers?''. One of the main reasons why these questions remain unanswered is a lack of observations of intermediate stages in the evolution towards gravitational-wave mergers, so-called gravitational-wave progenitors. One of the most sought for gravitational-wave progenitors of the last few years are single degenerate binaries -- binaries where one component is a compact object, and especially those with a black hole. These systems hold critical information on how black holes form. However, despite their importance, finding these single-degenerate binaries with black holes appears to be extremely challenging with our current methods. Instead of black holes, scientists discovered binaries that, at first sight, resemble a single-degenerate binary. However, an in depth analysis then shows that there is no black hole present in these systems but post-interaction products. These systems are now termed black-hole imposters. While not explicitly sought for, these post-interaction binaries are also crucial to unravel how massive binaries would evolve into gravitational-wave mergers. One typical post-interaction object is thought to be a Be star -- a rapidly rotating B-type star with a decretion disc. The availability of binary statistics and properties of Be binaries would make it possible to investigate in more detail how binaries interact. However, this information is currently unavailable due to challenges in their analysis. With these two questions in mind, this thesis focusses on finding a population of single-degenerate binaries in the Milky Way. For this purpose, we utilise a new state-of-the-art method for the discovery of single-degenerate binaries with a black hole. This method uses astrometric data from the Gaia space telescope. We predict that Gaia has the potential to increase the currently observed number of electromagnetic black holes by a factor of 10-20. Moreover, we show that these detections will allow us to constrain how black holes form. Furthermore, according to our predictions, the amount of black-hole imposters amongst the single-degenerate binaries detected with Gaia should be small compared to the true single-degenerate binaries. We also showed that our predictions can only be tested in future Gaia data releases. We also studied Be binaries with compact-object companions. First, we challenged the only reported Be star with a black-hole companion, bringing the confirmed amount of black holes around Be stars back to zero. Second, we perform an optical spectroscopic analysis of a sample of currently interacting single-degenerate binaries in the form of Be-X-ray binaries. With the orbital periods obtained from the X-rays, we can establish the most reliable methods and spectral lines to determine radial velocities for Be stars. This work will allow us to develop a good methodology for the analysis of the Be-star population, which will result in the necessary binary statistics and properties for the Be population. The two aspects addressed in this thesis should significantly improve our understanding of which and how massive binaries evolve into gravitational-wave mergers. We have laid the foundation to discover a population of one of the most wanted gravitational-wave progenitors, single-degenerate binaries with a black hole, and improved the analysis of Be stars. We also showed that there are currently no confirmed Be stars with a black hole companion.