  Hot subdwarfs represent a poorly understood late phase of stellar evolution.
While binary evolution plays an important role for B-subdwarfs (sdB), the
origin of the He dominated O-subdwarfs (He-sdO) is unknown. We search for
chemical signatures of their genesis by means of quantitative analyses of
visual and FUV spectra. Using Tlusty/Synspec to compute line blanketed NLTE
model atmospheres, Teff/log g and abundances have been derived for 4
prototypical He-sdO stars. Final models included
H,He,C,N,O,Ne,Mg,Al,Si,P,S,Fe&Ni. Because of the enrichment of either N or C,
models including these elements at the appropriate high abundance provide
sufficiently accurate approximations to the temperature stratification of full
models. No indications for binarity were found, neither radial velocity
variations nor photometric evidence. All stars have atmospheres almost free of
H and Teff =42000K to 47000K, and log g= 5.4 to 5.7. CD-31 4800 displays the
signatures of CNO burning, while heavier elements are subsolar by ~0.4 dex,
except Ne & Si being ~solar. The C-rich He-sdOs show a slightly subsolar
metallicity accompanied by N-enrichment and O-deficiency. Ne is enriched up to
a factor of ten with respect to the sun in LS IV+10 9. Stellar masses scatter
around the canonical helium core flash mass. The abundance pattern of CD-31
4800 is consistent with predictions of models for cold mergers of He-WDs except
for the low O abundance observed. Models for composite mergers predict
abundance pattern dissimilar to those determined for the C-rich stars.
[CW83]0904-02, though, may be a candidate for a composite He-WD merger, as it
rotates and is more massive than the other stars. New evolutionary models for
the hot flasher scenario predict abundance patterns similar to those determined
for the C-rich stars. Hence, C-rich He-sdO may result from late He flashes with
deep-mixing episodes. (abbreviated)
