2025RNAAS...9..347W

Theta Eridani (θ Eri) is a nearby $\rho =8\mathop{.}\limits^{^{\prime\prime} }3\leftrightarrow 425\,{\rm{au}}$ visual binary (A+B). The brighter and redder primary (A) is often referred to as a subgiant, but is, in fact, a double-lined spectroscopic binary. Here we report on a VLTI/GRAVITY interferometric observation in which the primary was spatially resolved into a close binary with a projected separation ρ = 1.20 mas ↔ 0.06 au while the secondary (B) is consistent with a single star. Isochrone fitting then implies masses M_Aa ≃ 2.39M_⊙, M_Ab ≃ 2.35M_⊙ and M_B ≃ 2.30M_⊙ and somewhat expanded radii R_Aa ≃ 3.9R_⊙, R_Ab ≃ 3.6R_⊙ and R_B ≃ 3.2R_⊙. The very close binary Aa+Ab can explain the TESS lightcurve of θ Eri with a periodicity of 4.1077 days through ellipsoidal modulations.