Context. It has recently been proposed that LB-1 is a binary system at 4 kpc consisting of a B-type star of 8 M☉ and a massive stellar black hole (BH) of 70 M☉. This finding challenges our current theories of massive star evolution and formation of BHs at solar metallicity.
Aims: Our objective is to derive the effective temperature, surface gravity, and chemical composition of the B-type component in order to determine its nature and evolutionary status and, indirectly, to constrain the mass of the BH.
Methods: We use the non-LTE stellar atmosphere code FASTWIND to analyze new and archival high-resolution data.
Results: We determine (Teff, log g) values of (14 000 ± 500 K, 3.50 ± 0.15 dex) that, combined with the Gaia parallax, imply a spectroscopic mass, from log g, of 3.2+2.1-1.9 M☉ and an evolutionary mass, assuming single star evolution, of 5.2+0.3-0.6 M☉. We determine an upper limit of 8 km s-1 for the projected rotational velocity and derive the surface abundances; we find the star to have a silicon abundance below solar, and to be significantly enhanced in nitrogen and iron and depleted in carbon and magnesium. Complementary evidence derived from a photometric extinction analysis and Gaia yields similar results for Teff and log g and a consistent distance around 2 kpc.
Conclusions: We propose that the B-type star is a slightly evolved main sequence star of 3-5 M☉ with surface abundances reminiscent of diffusion in late B/A chemically peculiar stars with low rotational velocities. There is also evidence for CN-processed material in its atmosphere. These conclusions rely critically on the distance inferred from the Gaia parallax. The goodness of fit of the Gaia astrometry also favors a high-inclination orbit. If the orbit is edge-on and the B-type star has a mass of 3-5 M☉, the mass of the dark companion would be 4-5 M☉, which would be easier to explain with our current stellar evolutionary models.