NGC 1850 BH1 is another stripped-star binary masquerading as a black
  hole
  We show that the radial velocity-variable star in the black hole candidate
NGC 1850 BH1 cannot be a normal $\approx 5\,M_{\odot}$ subgiant, as was
proposed, but is an overluminous stripped-envelope star with mass $\approx 1
M_{\odot}$. 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 $\approx 1.5\,M_{\odot}$. 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
$(2.5-5)\,M_{\odot}$ 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 a $\sim5\,M_{\odot}$ 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 black hole, NGC 1850 BH1 provides an interesting test case
for binary evolution models, particularly given its membership in a cluster of
known age.
