Discovery of a spectroscopic binary comprising two hot helium-rich subdwarfs

Helium-rich subdwarf B (He-sdB) stars form a very small fraction of the population of hot subdwarf stars in our Galaxy. They have been found both in the field of our Galaxy as well as in globular clusters. The evolution of these rare stars has recently been the subject of much debate involving both binary and single star evolution models. We report our serendipitous discovery that the prototype – PG 1544+488 – is a binary containing two low-mass helium-rich hot subdwarfs. This discovery challenges existing evolutionary models for He-sdB stars and suggests that they may be formed through close binary evolution and following the ejection of a common envelope.