  The recently claimed discovery of a massive
($M_\mathrm{BH}=68^{+11}_{-13}\,M_\odot$) black hole in the Galactic solar
neighborhood has led to controversial discussions because it severely
challenges our current view of stellar evolution. A crucial aspect for the
determination of the mass of the unseen black hole is the precise nature of its
visible companion, the B-type star LS V+22 25. Because stars of different mass
can exhibit B-type spectra during the course of their evolution, it is
essential to obtain a comprehensive picture of the star to unravel its nature
and, thus, its mass. To this end, we study the spectral energy distribution of
LS V+22 25 and perform a quantitative spectroscopic analysis that includes the
determination of chemical abundances for He, C, N, O, Ne, Mg, Al, Si, S, Ar,
and Fe. Our analysis clearly shows that LS V+22 25 is not an ordinary main
sequence B-type star. The derived abundance pattern exhibits heavy imprints of
the CNO bi-cycle of hydrogen burning, that is, He and N are strongly enriched
at the expense of C and O. Moreover, the elements Mg, Al, Si, S, Ar, and Fe are
systematically underabundant when compared to normal main-sequence B-type
stars. We suggest that LS V+22 25 is a stripped helium star and discuss two
possible formation scenarios. Combining our photometric and spectroscopic
results with the Gaia parallax, we infer a stellar mass of
$1.1\pm0.5\,M_\odot$. Based on the binary system's mass function, this yields a
minimum mass of $2-3\,M_\odot$ for the compact companion, which implies that it
may not necessarily be a black hole but a massive neutron- or main sequence
star. The star LS V+22 25 has become famous for possibly having a very massive
black hole companion. However, a closer look reveals that the star itself is a
very intriguing object. Further investigations are necessary for complete
characterization of this object.

Solar and Stellar Astrophysics