Spectral analysis of the low-gravity extreme helium stars LSS 4357 LS II+33 degrees 5 and LSS 99

We have carried out quantitative analyses of three very low surface gravity extreme helium stars with very similar spectra. Their effective temperatur es of similar to 16 000 K fill a gap in a nearly continuous sequence of ext reme helium stars all having similar luminosity-to-mass ratios, but extendi ng from effective temperatures around 12 000 K to more than 20 000 K. Becau se of the low surface gravities and extremely rich line spectra, the model atmosphere calculations have been reviewed, and large-scale spectral synthe sis techniques have been introduced to the analyses for the first time. In addition to the high carbon and nitrogen abundances usually seen in extreme helium stars, two of the programme stars have extremely high oxygen abunda nces, comparable with or greater than their carbon abundances.