text: 12,102,1503700997566308352,952949678533849088,Kareem El-Badry,"New paper! We study two “mass gap” black hole candidates in binaries with red giant stars, “the Unicorn” and “the Giraffe”. 1/n <LINK> <LINK> We used spectral disentangling to search for possible luminous companions (as opposed to BHs) to the giants. We found them! 2/n <LINK> What that means, roughly, is that two luminous stars do (much) a better job fitting the spectra than one. <LINK> In both systems, the disentangled spectra of the companions look like subgiant stars (i.e., cooler and larger than main-sequence stars; warmer and smaller than giants). 4/n <LINK> Because these subgiants are cooler than main-sequence stars of similar mass, they are faint in the UV and consistent with the observed spectral energy distributions and UV limits. 5/n <LINK> We can measure the masses of the giants from the observed ellipsoidal variation. In both systems, they are 0.3-0.5 Msun. This is very low for a giant, and implies most of their initial mass was stripped off by a companion. 6/n <LINK> We can also measure the masses of the subgiants dynamically. The dynamically-inferred values (1.8 and 2.8 Msun) are in reasonably good agreement with what we'd estimate from their temperature and luminosity. 7/n <LINK> We used binary evolution models to investigate how these systems formed and how they'll evolve in the future. We think the giant are almost completely stripped of their envelopes and will soon contract to become low-mass helium white dwarfs. 8/n <LINK> This scenario (and the component masses) is almost identical to how we think Regulus, the ~20th brightest star in the sky, formed. It's a main-sequence star with a helium white dwarf companion in a wide orbit. <LINK> 9/n The fact that the companions are subgiants (i.e, off the main sequence) implies that either the initial mass ratio was very close to 1 (like, q&gt;0.99), or that the companions are temporarily inflated due to rapid accretion. 9/ The second possibility is particularly exciting, but it will take more work (ideally a population model of interacting giant binaries) to test it. The Unicorn and Giraffe join a growing population of mass-transfer binaries recently observed at various stages of the stripping process. Several of these other objects have also been previously interpreted as BHs. <LINK> Summary: stellar-mass BHs are small needles in a very large haystack. But they haystack contains a lot of other interesting stuff! n/n",https://arxiv.org/abs/2203.06348,"We analyze two binary systems containing giant stars, V723 Mon (""the Unicorn"") and 2M04123153+6738486 (""the Giraffe""). Both giants orbit more massive but less luminous companions, previously proposed to be mass-gap black holes. Spectral disentangling reveals luminous companions with star-like spectra in both systems. Joint modeling of the spectra, light curves, and spectral energy distributions robustly constrains the masses, temperatures, and radii of both components: the primaries are luminous, cool giants ($T_{\rm eff,\,giant} = 3,800\,\rm K$ and $4,000\,\rm K$, $R_{\rm giant}= 22.5\,R_{\odot}$ and $25\,R_{\odot}$) with exceptionally low masses ($M_{\rm giant} \approx 0.4\,M_{\odot}$) that likely fill their Roche lobes. The secondaries are only slightly warmer subgiants ($T_{\rm eff,\,2} = 5,800\,\rm K$ and $5,150\,\rm K$, $R_2= 8.3\,R_{\odot}$ and $9\,R_{\odot}$) and thus are consistent with observed UV limits that would rule out main-sequence stars with similar masses ($M_2 \approx 2.8\,M_{\odot}$ and $\approx 1.8\,M_{\odot}$). In the Unicorn, rapid rotation blurs the spectral lines of the subgiant, making it challenging to detect even at wavelengths where it dominates the total light. Both giants have surface abundances indicative of CNO processing and subsequent envelope stripping. The properties of both systems can be reproduced by binary evolution models in which a $1-2\,M_{\odot}$ primary is stripped by a companion as it ascends the giant branch. The fact that the companions are also evolved implies either that the initial mass ratio was very near unity, or that the companions are temporarily inflated due to rapid accretion. The Unicorn and Giraffe offer a window into into a rarely-observed phase of binary evolution preceding the formation of wide-orbit helium white dwarfs, and eventually, compact binaries containing two helium white dwarfs. ","Unicorns and Giraffes in the binary zoo: stripped giants with subgiant