2018ApJ...858..103F

We report on a fossil stellar merger for the subgiant primary of the visual binary HR 3750. The subgiant leads to an age τ ≃ 3.2 Gyr for a mass M_A = 1.39 ± 0.09 M_☉, in contradiction with its low iron-to-magnesium abundance that classifies it as a Population II (thick-disk) star. Upon the assumption of an ancient source, and since there appears to be no inner Aa-Ab subsystem for HR 3750, the mass of the subgiant primary can only be understood in terms of a merger with a former tertiary component. In a mass conserving scenario, and with M_Aa = 1.03 ± 0.03 M_☉ as the likely progenitor mass of the primary, the mass of the accreted companion is suggestive of an M dwarf at M_Ab = 0.36 ± 0.03 M_☉.