We present EVR-CB-001, the discovery of a compact binary with an extremely
low mass (.21±0.05M⊙) helium core white dwarf progenitor (pre-He
WD) and an unseen low mass (.32±0.06M⊙) helium white dwarf (He
WD) companion. He WDs are thought to evolve from the remnant helium-rich core
of a main-sequence star stripped during the giant phase by a close companion.
Low mass He WDs are exotic objects (only about .2% of WDs are thought to be
less than .3 M⊙), and are expected to be found in compact binaries.
Pre-He WDs are even rarer, and occupy the intermediate phase after the core is
stripped, but before the star becomes a fully degenerate WD and with a larger
radius (≈.2R⊙) than a typical WD. The primary component of
EVR-CB-001 (the pre-He WD) was originally thought to be a hot subdwarf (sdB)
star from its blue color and under-luminous magnitude, characteristic of sdBs.
The mass, temperature (Teff=18,500±500K), and surface gravity
(log(g)=4.96±0.04) solutions from this work are lower than values for
typical hot subdwarfs. The primary is likely to be a post-RGB, pre-He WD
contracting into a He WD, and at a stage that places it nearest to sdBs on
color-magnitude and Teff-log(g) diagrams. EVR-CB-001 is expected to
evolve into a fully double degenerate, compact system that should spin down and
potentially evolve into a single hot subdwarf star. Single hot subdwarfs are
observed, but progenitor systems have been elusive.Comment: 14 pages, 11 figures. Published in The Astrophysical Journa