Subdwarf O star

[3] The structure of a subdwarf O star is believed to be a carbon and oxygen core surrounded by a helium burning shell.

[2] In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung-Russell diagram.

However, there are variations with enhancement in concentration of even numbered elements such as carbon, oxygen, neon, silicon, magnesium or iron.

An alternate theory is that sdOs have been formed by coalescing two white dwarfs.

This could happen from a close binary that decays due to gravitational waves.