Abstract B-type stars with masses up to about 8 M _⊙ are the progenitors of the heavier white dwarfs yet our knowledge about their multiplicity is still incomplete. The nearby subgiant HIP 92041 = ϕ Sagittarii has had claims of binarity in the past but they have all been refuted as spurious. In this report we show that ϕ Sgr is a binary after all based on a VLTI/GRAVITY interferometric observation. The companion has a K band flux ratio of 6% and a projected separation ρ = 17.7 mas ↔ 1.3 au. The inferred isochrone masses 3.5M _⊙ + 1.6M _⊙ make ϕ Sgr a potential progenitor for a white dwarf binary with a combined mass exceeding the Chandrasekhar limit.