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Hydrogen deficient stars include the cool R CrB variable (RCBs) andhydrogen-deficient carbon (HdCs) giants through extreme helium stars (EHes) tothe very hot helium-rich subdwarfs (He-sdO and O(He) stars) and white dwarfs.With surfaces rich in helium, nitrogen and carbon, their origins have beenidentified with the merger of two white dwarfs. Using Gaia to focus on theEHes, we aim to identify progenitor populations and test the evolution models.Gaia DR3 measurements and ground-based radial velocities have been used tocompute Galactic orbits using galpy. Each orbit has been classified bypopulation; EHe stars are found in all of the thin disk, thick disk, halo andbulge, as are RCB, HdC and He-sdO stars. Spectral energy distributions wereconstructed for all EHes, to provide angular diameters, and hence radii andluminosities. The EHes fall into two luminosity groups divided at L ~ 2500solar L. This supports theory for the origin of EHes, and is the strongestconfirmation so far in terms of luminosity. The lower luminosity EHescorrespond well with the post-merger evolution of a double helium white dwarfbinary. Likewise, the higher luminosity EHes match the post-merger evolution ofa carbon/oxygen plus helium white dwarf binary. In terms of parent populations,current models predict that double white dwarf mergers should occur in allGalactic populations, but favour mergers arising from recent star formation(i.e. thin disk), whereas the statistics favour an older epoch (i.e. thickdisk).

The distribution, kinematics and luminosities of extreme helium stars as probes of their origin and evolution