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+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- 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 (Jeffery & Heber 1992), these seven stars form a clear sequence of extreme helium stars with a luminosity to mass ratio (solar units). The significance of this sequence and why it appears to be distinct from a number of helium stars with , will be explored in a concluding paper.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998