2022MNRAS.511L..24E

We show that the radial velocity variable star in the black hole (BH) candidate NGC 1850 BH1 cannot be a normal ~5M_☉_ subgiant, as was proposed, but is an overluminous stripped-envelope star with mass ≃1 M_☉. The result follows directly from the star's observed radius and the orbital period-density relation for Roche lobe-filling stars: The star's density, as constrained by the observed ellipsoidal variability, is too low for its mass to exceed ~1.5 M_☉. This lower mass significantly reduces the implied mass of the unseen companion and qualitative interpretation of the system, such that a normal main-sequence companion with mass 2.5-5 M_☉ is fully consistent with the data. We explore evolutionary scenarios that could produce the binary using MESA and find that its properties can be matched by models in which an ~5M_☉_ primary loses most of its envelope to a companion and is observed in a bloated state before contracting to become a core helium burning sdOB star. This is similar to the scenario proposed to explain the binaries LB-1 and HR 6819. Though it likely does not contain a BH, NGC 1850 BH1 provides an interesting test case for binary evolution models, particularly given its membership in a cluster of known age.