2025RNAAS...9..225W

Chi Lupi (χ Lup) is a nearby a = 0.20 au, 2.8M_⊙ + 1.9M_⊙ close binary and the prototype for the class of chemically peculiar mercury-manganese (HgMn) stars. There has been evidence for the presence of a third, outer companion based on variations of the systemic velocity of the inner binary but up to now it had evaded detection. Here we report on a VLTI/GRAVITY observation in which we detected an M_C = 0.79M_⊙ red dwarf companion at a projected separation ρ ≃ 120 mas ↔ 7.5 au from the close binary, confirming χ Lup as a very compact triple system. The outer companion explains the eROSITA X-ray emission of χ Lup and hints that the inner binary was probably hardened by interacting with a circumbinary disk from which the outer companion also formed.