1998A&A...329.1019D

LSS 3184 is a hydrogen-deficient, early B-type giant, recently found to pulsate with a period of 2.5 hours. Its photospheric parameters have been derived from optical high-resolution spectra by the method of fine analysis. The principal results are T_eff=23300 K, logg=3.35, n_H/n_He≲0.00015, n_C/n_He=0.003, n_N/n_He=0.0005, and n_O/n_He=0.0003. Hydrogen is extremely deficient. The effective temperature is consistent with broad-band visual and ultraviolet spectrophotometry and an extinction E_B–V∼0.27. Its previous evolution is reflected in the chemistry of the atmosphere, which contains enriched nitrogen from CNO-cycle hydrogen burning, and carbon from 3α helium burning. Thus LSS3184 is a true extreme helium star with a composition similar to BD-9 4395. With T_eff, logg, and pulsation properties very similar to the C-poor and N-rich helium star V652 Her, evolutionary mechanisms which can result in very different surface compositions for the two stars must be examined.