Hidden Companions to Intermediate-mass Stars. Upgraded Multiplicity 2 → 3. XXX. Discovery of a 0.79<i>M</i> _⊙, 7.5 au Outer Companion to the 0.20 au HgMn Binary Chi Lupi*

Abstract Chi Lupi ( χ Lup) is a nearby a = 0.20 au, 2.8 M ⊙ + 1.9 M ⊙ 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 = 0.79 M ⊙ 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.