article published yes Johanna Müller-Horn author Varsha Ramachandran author Kareem El-Badry author Andreas A.C. Sander author Julia K Bodensteiner author 48162413-c59a-11ee-a1af-dbba36c8a756 Douglas R. Gies author Ylva Louise Linsdotter Götberg author d0648d0c-0f64-11ee-a2e0-dd0faa2e4f7d0000-0002-6960-6911 Thomas Rivinius author Tomer Shenar author Elisa C. Schösser author Luqian Wang author Allyson Bieryla author Lars A. Buchhave author David W. Latham author YlGo department The Galactic Be star binary MWC 656 was long considered the only known Be star+black hole (BH) system, making it a critical benchmark for models of massive binary evolution and for the expected X-ray emission of Be+BH binaries. However, recent dynamical measurements cast doubt on the presence of a BH companion. We present new multi-epoch ultraviolet spectroscopy from the Hubble Space Telescope, combined with high-resolution optical spectra, to reassess the nature of the companion. The far-ultraviolet spectra reveal high-ionisation features – including prominent N v and He ii lines – which are absent in the spectra of normal Be stars and are indicative of a hot, luminous companion. Spectral modelling shows that these features cannot originate from the Be star or from an accretion disc around a compact object. Instead, we find that the data are best explained by a hot (Teff ≈ 85 kK), compact, hydrogen-deficient star with strong wind signatures, consistent with an intermediate-mass stripped star. Our revised orbital solution and composite spectroscopic modelling yield a companion mass of M2 = 1.48+0.55 −0.46 M, definitively ruling out a BH and disfavouring a white dwarf. MWC 656 thus joins the growing class of Be + stripped star binaries. The system’s unusual properties – including a high companion temperature and wind strength – extend the known parameter space of such binaries. The continued absence of confirmed OBe+BH binaries in the Galaxy highlights a growing tension with population synthesis models. https://research-explorer.ista.ac.at/download/22816/22891/2026_AstronomyAstrophysics_MuellerHorn.pdf application/pdfno https://creativecommons.org/licenses/by/4.0/ EDP Sciences2026 eng 0004-6361 1432-0746 2601.1440310.1051/0004-6361/202557960 708 Müller-Horn, Johanna, Varsha Ramachandran, Kareem El-Badry, Andreas A.C. Sander, Julia K Bodensteiner, Douglas R. Gies, Ylva Louise Linsdotter Götberg, et al. “Ultraviolet Spectroscopy Reveals a Hot and Luminous Companion to the Be Star+black Hole Candidate MWC 656.” <i>Astronomy &#38; Astrophysics</i>. EDP Sciences, 2026. <a href="https://doi.org/10.1051/0004-6361/202557960">https://doi.org/10.1051/0004-6361/202557960</a>. Müller-Horn J, Ramachandran V, El-Badry K, Sander AAC, Bodensteiner JK, Gies DR, Götberg YLL, Rivinius T, Shenar T, Schösser EC, Wang L, Bieryla A, Buchhave LA, Latham DW. 2026. Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656. Astronomy &#38; Astrophysics. 708, A187. J. Müller-Horn, V. Ramachandran, K. El-Badry, A.A.C. Sander, J.K. Bodensteiner, D.R. Gies, Y.L.L. Götberg, T. Rivinius, T. Shenar, E.C. Schösser, L. Wang, A. Bieryla, L.A. Buchhave, D.W. Latham, Astronomy &#38; Astrophysics 708 (2026). Müller-Horn, J., Ramachandran, V., El-Badry, K., Sander, A. A. C., Bodensteiner, J. K., Gies, D. R., … Latham, D. W. (2026). Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656. <i>Astronomy &#38; Astrophysics</i>. EDP Sciences. <a href="https://doi.org/10.1051/0004-6361/202557960">https://doi.org/10.1051/0004-6361/202557960</a> Müller-Horn J, Ramachandran V, El-Badry K, et al. Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656. <i>Astronomy &#38; Astrophysics</i>. 2026;708. doi:<a href="https://doi.org/10.1051/0004-6361/202557960">10.1051/0004-6361/202557960</a> Müller-Horn, Johanna, et al. “Ultraviolet Spectroscopy Reveals a Hot and Luminous Companion to the Be Star+black Hole Candidate MWC 656.” <i>Astronomy &#38; Astrophysics</i>, vol. 708, A187, EDP Sciences, 2026, doi:<a href="https://doi.org/10.1051/0004-6361/202557960">10.1051/0004-6361/202557960</a>. J. Müller-Horn <i>et al.</i>, “Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656,” <i>Astronomy &#38; Astrophysics</i>, vol. 708. EDP Sciences, 2026. 228162026-09-06T22:01:57Z2026-09-09T13:11:09Z