MWC 656 was classified as the first Be star orbiting a stellar-mass black hole — a system where a rapidly rotating emission-line star is paired with an invisible compact companion heavy enough to be a black hole. The classification was based on radial velocity measurements that implied a companion mass above three solar masses.
Hubble ultraviolet spectroscopy overturns this. The companion is visible in the UV: a hot, compact, hydrogen-deficient star with an effective temperature of approximately 85,000 K and strong stellar wind signatures. Its mass is approximately 1.5 solar masses — well below the minimum for a black hole and inconsistent with a white dwarf classification.
The object is a stripped star: what remains after a more massive star lost its hydrogen envelope to its companion. Stripped stars are predicted to be common products of binary stellar evolution, but they are difficult to detect because they are faint in the optical and overwhelmed by their companions' light. The UV reveals them because their extreme temperatures produce most of their radiation at short wavelengths.
The reclassification creates a tension with population synthesis models. These models predicted that Be + black hole binaries should be relatively common in the Milky Way. MWC 656 was the poster child. Its demotion to a Be + stripped star system means the predicted population of stellar-mass black holes in Be binaries may be overestimated, or the signature used to identify them is less reliable than assumed.
The black hole was an inference from incomplete data. The UV, which nobody had looked at, contained the actual answer.