1. Liu, J. et al. A wide star–black-hole binary system from radial-velocity measurements. Nature 575, 618–621 (2019).

2. Belczynski, K. et al. The formation of a 70-M ☉ black hole at high metallicity. Astrophys. J. 890, 113–859 (2020).

3. Groh, J. H. et al. Massive black holes regulated by luminous blue variable mass loss and magnetic fields – implications for the progenitor of LB-1. Preprint at http://arXiv.org/abs/1912.00994 (2019).

4. Ziółkowski, J. Evolutionary constraints on the masses of the components of the HDE 226868/Cyg X-1 binary system. Mon. Not. R. Astron. Soc. 358, 851–859 (2005).

5. Casares, J. et al. A Be-type star with a black-hole companion. Nature 505, 378–381 (2014).

6. Abbott, B. P. et al. GWTC-1: a gravitational-wave transient catalog of compact binary mergers observed by LIGO and Virgo during the first and second observing runs. Phys. Rev. X 9, 031040 (2019).

7. Raskin, G. et al. HERMES: a high-resolution fibre-fed spectrograph for the Mercator telescope. Astron. Astrophys. 526, A69 (2011).

8. El-Badry, K. & Quataert, E. Not so fast: LB-1 is unlikely to contain a 70 M ☉ black hole. Mon. Not. R. Astron. Soc. 493, L22–L27 (2020).

9. Tkachenko, A. Grid search in stellar parameters: a software for spectrum analysis of single stars and binary systems. Astron. Astrophys. 581, A129 (2015).

10. Simón-Díaz, S. et al. A detailed non-LTE analysis of LB-1: revised parameters and surface abundances. Astron. Astrophys. 634, L7 (2020).

11. Lanz, T. & Hubeny, I. A grid of non-LTE line-blanketed model atmospheres of O-type stars. Astrophys. J. Suppl. Ser. 146, 417–441 (2003).

12. Schneider, F. R. N. et al. Bonnsai: a Bayesian tool for comparing stars with stellar evolution models. Astron. Astrophys. 570, A66 (2014).

13. Brott, I. et al. Rotating massive main-sequence stars. I. Grids of evolutionary models and isochrones. Astron. Astrophys. 530, A115 (2011).

14. Condori, C. A. H. et al. The study of unclassified B[e] stars and candidates in the Galaxy and Magellanic Clouds. Mon. Not. R. Astron. Soc. 488, 1090–1110 (2019).