Hidden Companions to Intermediate-mass Stars. Upgraded Multiplicity 2 → 3. XXIX. Discovery of a 1.65M ⊙, 1.0 au Companion to Xi1 Lupi*

Xi1 Lupi ( ξ 1 Lup) is a nearby star (A) that forms a 10 . ″ 2 ↔ 660 au intermediate-mass visual binary with ξ 2 Lup (B). The fact that the primary A is both brighter and redder than the secondary B gives rise to an age contradiction for the two components. Here we report on a VLTI/GRAVITY observation that revealed that ξ 1 Lup is actually a close binary: the companion has a K band flux ratio of 59.1% ± 0.2% at a projected separation ρ = 15.88 mas ↔ 1.0 au. From isochrone fitting we find masses M Aa ≃ 2.0 M ⊙ , M Ab ≃ 1.65 M ⊙ and M B ≃ 2.0 M ⊙ and a system age t ≃ 450–500 Myr; the binarity of ξ 1 Lup hence solves the age contradiction. The triple system might be a progenitor for a triple white dwarf system in about 1.5 Gyr depending on the de-projected separation of Aa+Ab.