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Astron. Astrophys. 340, 476-482 (1998)

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6. Conclusions

A quantitative analysis has provided photospheric parameters and abundances for three extreme helium stars. These have been confirmed to have effective temperatures of around 16 000 K and surface gravities close to the limit at which model atmospheres can currently be calculated in hydrostatic and local thermodynamic equilibrium. From Tables 1 and 2 and Fig. 1 it may be seen that all three stars have very similar properties. Indeed, LSS 4357 and LS II+[FORMULA]5 are almost twins. No other pair of helium stars studied to date are so similar. They also have the highest oxygen abundance of any extreme helium star, previously the low-gravity helium stars DY Cen and LSE 78 held this record. In the log g-[FORMULA] diagram (Fig. 2), the programme stars lie directly between these stars and HD 168476 (Walker & Schönberner 1981), itself a moderately oxygen rich star. Together with BD[FORMULA] (Jeffery & Heber 1992), these seven stars form a clear sequence of extreme helium stars with a luminosity to mass ratio [FORMULA] (solar units). The significance of this sequence and why it appears to be distinct from a number of helium stars with [FORMULA], will be explored in a concluding paper.

[FIGURE] Fig. 2. The [FORMULA] diagram for extreme helium stars showing the position of the programme stars, the hydrogen main sequence (H-MS), the classical Eddington limit for radiative stability, and loci of constant L/M (solar units: dotted lines). Other EHes referred to by name in the text are also labelled.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998