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About a quarter of all post-asymptotic giant branch (AGB) stars arehydrogen-deficient. Stellar evolutionary models explain the carbon-dominatedH-deficient stars by a (very) late thermal pulse scenario where thehydrogen-rich envelope is mixed with the helium-rich intershell layer.Depending on the particular time at which the final flash occurs, the entirehydrogen envelope may be burned. In contrast, helium-dominated post-AGB starsand their evolution are yet not understood. A small group of very hot,helium-dominated stars is formed by O(He)-type stars. We performed a detailedspectral analysis of ultraviolet and optical spectra of four O(He) stars bymeans of state-of-the-art non-LTE model-atmosphere techniques. We determinedeffective temperatures, surface gravities, and the abundances of H, He, C, N,O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-lossrates of the O(He) stars, we found that they do not exhibit enhanced mass-loss.The comparison with evolutionary models shows that the status of the O(He)stars remains uncertain. Their abundances match predictions of a double heliumwhite dwarf merger scenario, suggesting that they might be the progeny of thecompact and of the luminous helium-rich sdO-type stars. The existence ofplanetary nebulae that do not show helium enrichment around every other O(He)star, precludes a merger origin for these stars. These stars must have formedin a different way, for instance via enhanced mass-loss during their post-AGBevolution or a merger within a common-envelope (CE) of a CO-WD and a red giantor AGB star. A helium-dominated stellar evolutionary sequence exists, that maybe fed by different types of mergers or CE scenarios. It appears likely, thatall these pass through the O(He) phase just before they become white dwarfs.

On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars