J/A+A/708/A187ivo://CDS.VizieR/J/A+A/708/A187CDSMueller-Horn J.Ramachandran V.El-Badry K.Sander A.A.C.,Bodensteiner J.Gies D.R.Goetberg Y.Rivinius T.Shenar T.,Schoesser E.C.Wang L.Bieryla A.Buchhave L.A.Latham D.W.2026-05-04T07:43:19Z2026-04-08T12:31:17ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frspectroscopic-binary-starsspectroscopyultraviolet-astronomyearly-type-starsThe 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 NV and HeII 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 (T_eff_~85kK), 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 M_2_=1.48^+0.55^_-0.46_M_{sun}_, 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.2026A&A...708A.187Mhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/708/A187CatalogResearchIsServedByTAP VizieR generic servicehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/A+A/708/A187https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/A+A/708/A187http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/A+A/708/A187https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/A+A/708/A187https://vizier.iucaa.in/viz-bin/votable?-source=J/A+A/708/A187http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/A+A/708/A187GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapUVdefaultJ/A+A/708/A187/listList of spectrarecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintFileNameName of the spectrum file within subdirectorymeta.id;meta.filecharTitleTitlemeta.titlecharspShow the spectrummeta.ref.urlchar