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STELLAR PROPERTIES

The characteristic surface properties, effective temperature $T$, surface gravity $g$ and composition $X$, of EHes have been summarized by Jeffery (1996). The distribution of EHes in the $T-g$ plane is shown in Fig. 1 together with the location of other H-deficient stars, notably the putatively related RCrB stars. It may be seen that they lie roughly along a locus given by $\log L/M\sim 4$, which also corresponds to the evolution track of post-AGB stars, contracting towards the white dwarf track. This has guided most of discussion of their evolutionary status within the context of post-AGB evolution.

The surface compositions of EHes vary considerably from star to star, but may be briefly summarized as follows.

Hydrogen: ( $-4.6 < \log$    $n_{\rm H}$$<-0.8$) The fact that some hydrogen is found in the atmospheres of nearly all EHes indicates that some remnant of the outermost hydrogen-rich layers of the progenitor has been retained.

Helium: ( $0.9<$  $n_{\rm He}$$<1.0$) By definition, helium dominates the atmospheric abundances.

Nitrogen: ( $0.4<[N/Fe]<1.2$) Nitrogen is enriched in most EHes, implying that helium has been produced by CNO cycling. If all nitrogen comes from such a source, it should reflect the total C+N(+O) abundances in the progenitor. This is supported by a correlation between N and Fe abundances in EHes.

Carbon: ( $0.9<[C/Fe]<2.0$) Carbon is substantially enriched in nearly all EHes, indicating the presence of material processed through $3\alpha$ helium burning.

Oxygen: ( $-0.6<[O/Fe]<0.8$) Since $n_{\rm N}$ indicates the destruction of oxygen, nearly all observed oxygen has probably been produced by $\mbox{$^{12}{\rm C}$}$$(\alpha,\gamma)$$\mbox{$^{16}{\rm O}$}$. The O/C ratio should therefore provide a diagnostic of conditions in the C-rich layers of the progenitor. Observed O/C ratios do not approach the value expected in CO cores, except in two cases (DYCen and LSE78).

Iron: The iron abundances fall approximately into two groups (with the exceptions of DYCen and HD144941) clustered about $[Fe]\sim0.0\pm0.1$ and $[Fe]\sim-0.9\pm0.2$. It remains to be seen whether the bimodal distribution of iron abundances persists to the complete EHe sample.

Consequently, any model for the origin of EHes must result in a surface mixture which includes a remnant of the hydrogen envelope, predominantly CNO-processed helium and a significant quantity of $3\alpha$ and $\mbox{$^{12}{\rm C}$}$$+\alpha$ products, as well as reproducing the overall dimensions such as luminosity, mass and effective temperature.