The Origin of Helium-Rich Subdwarf O Stars

The origin of helium-rich subdwarf O stars (He-sdO) is still unclear. Significant progress has been made recently in placing them in the Hertzsprung–Russell diagram. However, the question of their progenitors is still unsolved. It is unclear which types of stars evolve into He-sdOs and whether they are formed by all galactic populations. Here I will present results of a radial velocity (RV) survey of He-sdOs, which will be used to constrain the binary fraction and evaluate population membership.