2025RNAAS...9..184W

Xi1 Lupi (ξ¹ Lup) is a nearby star (A) that forms a $10\mathop{.}\limits^{^{\prime\prime} }2\leftrightarrow 660\,{\rm{au}}$ intermediate-mass visual binary with ξ² 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 ξ¹ 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 ≃ 2.0M_⊙, M_Ab ≃ 1.65M_⊙ and M_B ≃ 2.0M_⊙ and a system age t ≃ 450-500 Myr; the binarity of ξ¹ 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.