@article{22816,
  abstract     = {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.},
  author       = {Müller-Horn, Johanna and Ramachandran, Varsha and El-Badry, Kareem and Sander, Andreas A.C. and Bodensteiner, Julia K and Gies, Douglas R. and Götberg, Ylva Louise Linsdotter and Rivinius, Thomas and Shenar, Tomer and Schösser, Elisa C. and Wang, Luqian and Bieryla, Allyson and Buchhave, Lars A. and Latham, David W.},
  issn         = {1432-0746},
  journal      = {Astronomy & Astrophysics},
  publisher    = {EDP Sciences},
  title        = {{Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656}},
  doi          = {10.1051/0004-6361/202557960},
  volume       = {708},
  year         = {2026},
}