Hidden Companions to Intermediate-mass Stars. XXVIII. Uncovering a 0.62 <i>M</i> _⊙, 0.18 au Companion to Lambda Aquilae*

Abstract Lambda Aquilae ( λ Aql) is a nearby late B-type star with somewhat contradictory evidence of being a binary system. Here we report on a VLTI/GRAVITY interferometric observation in which we directly detected a close companion with a K band flux ratio of 2.206% at projected separation ρ = 4.72 mas ↔ 0.18 au. Isochrone fitting implies masses M A ≃ 3.0 M ⊙ and M B ≃ 0.62 M ⊙ and an age of 90 Myr. The secondary can explain the X-ray emission of λ Aql. The primary should easily fill its Roche lobe in the red giant branch and trigger a common envelope event, which will most likely result in a merger.