  We report the discovery of HE 0430-2457, the first extremely low-mass
pre-white dwarf (ELM pre-WD) in a long period binary (P = 771 +- 3 d). The
spectroscopic 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 core
He-burning hot subdwarf B stars. By comparing the spectroscopic parameters of
the K-type companion to stellar models, and using the mass ratio, the mass of
the hot primary is determined to be 0.23 Msun. Given that this is too low for
core He-burning, the primary in HE 0430-2457 is not an EHB star but a pre WD of
the ELM type. As the lifetime of ELM pre-WDs in this region of the HR diagram
populated by EHBs is thought to be very short, they are not considered to be
part of the observed EHBs. However, the discovery of this system indicates that
the percentage of ELM pre-WDs in the observed EHB population might be higher
than previously thought. Binary evolution models indicate that HE 0430-2457 is
likely formed by a merger of the inner binary in a hierarchical triple system.
