Observational analysis reveals a helium-shell burning blue horizontal branch star in the Feige 64 binary system, indicating common envelope evolution drives formation.
Observationally, blue horizontal branch (BHB) stars are defined as hot stars occupying a characteristic region between the extreme blue horizontal branch and RR Lyrae variables in the Hertzsprung–Russell diagram. Most of them are interpreted as stripped core-helium-burning stars, but the role of binary interaction in their formation remains unclear. Here, we report the discovery of a metal-rich BHB star in a 0.82628-day binary system (Feige 64) comprising a 0.35 ± 0.03 M⊙ BHB star and a likely 1.26 ± 0.17 M⊙ white dwarf (WD). The BHB star has an effective temperature of 15, 524 ± 310 K and a luminosity of 39.7 ± 4.1 L⊙. Stellar evolution modelling indicates that it is a helium-shell-burning star produced through the common-envelope channel, retaining a hydrogen-rich envelope that is more massive than previously thought for low-mass stars. This finding provides direct evidence for binary interaction in the formation of BHB stars, offering a fresh perspective on interpreting this emerging population.
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