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We report the discovery of HE 0430-2457, the first extremely low-masspre-white dwarf (ELM pre-WD) in a long period binary (P = 771 +- 3 d). Thespectroscopic parameters of the primary are determined to be Teff = 26200 +-1500 K and logg = 5.40 +- 0.35, placing it in the region occupied by coreHe-burning hot subdwarf B stars. By comparing the spectroscopic parameters ofthe K-type companion to stellar models, and using the mass ratio, the mass ofthe hot primary is determined to be 0.23 Msun. Given that this is too low forcore He-burning, the primary in HE 0430-2457 is not an EHB star but a pre WD ofthe ELM type. As the lifetime of ELM pre-WDs in this region of the HR diagrampopulated by EHBs is thought to be very short, they are not considered to bepart of the observed EHBs. However, the discovery of this system indicates thatthe percentage of ELM pre-WDs in the observed EHB population might be higherthan previously thought. Binary evolution models indicate that HE 0430-2457 islikely formed by a merger of the inner binary in a hierarchical triple system.

HE 0430-2457: a post merger extremely low-mass pre-white dwarf in a wide binary posing as an extreme horizontal branch star