The donor stars of ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are the most extreme X-ray binaries in the Universe. As the large majority of ULXs are extragalactic objects and their optical emission is in most cases dominated by the accretion disc, spectroscopic identification of ULX donors is challenging. Many ULXs are believed to have high mass donor stars as they are found in or near star forming regions. This makes them possible progenitors for the black hole mergers discovered by LIGO. To learn where they fit in the picture of massive binary star evolution, knowledge of the donor stars in these systems is crucial. Detecting stellar absorption lines is also necessary to obtain dynamical mass measurements, which is the only direct way to determine the mass of any black hole accretors in ULXs. Of the five ULX donor stars that have been identified spectroscopically, three are red supergiants discovered in our near-IR survey of nearby ULXs. The photometric part of this survey is now finished and our spectroscopic follow-up campaign, to classify the counterparts and monitor the ones that are confirmed to be stellar, is well underway. I will show the latest results of this campaign and discuss the opportunities afforded by upcoming facilities such as JWST and 30-m class telescopes.