  Hot, compact, hydrogen-deficient pre-white dwarfs (pre-WDs) with effective
temperatures of Teff > 70,000 K and a surface gravity of 5.0 < log g < 7.0 are
rather rare objects despite recent and ongoing surveys. It is believed that
they are the outcome of either single star evolution (late helium-shell flash
or late helium-core flash) or binary star evolution (double WD merger). Their
study is interesting because the surface elemental abundances reflect the
physics of thermonuclear flashes and merger events. Spectroscopically they are
divided in three different classes, namely PG1159, O(He), or He-sdO. We present
a spectroscopic analysis of five such stars that turned out to have atmospheric
parameters in the range Teff = 70,000-80,000 K and log g = 5.2-6.3. The three
investigated He-sdOs have a relatively high hydrogen mass fraction (10%) that
is unexplained by both single (He core flash) and binary evolution (He-WD
merger) scenarios. The O(He) star JL9 is probably a binary helium-WD merger,
but its hydrogen content (6%) is also at odds with merger models. We found that
RL 104 is the 'coolest' (Teff = 80,000 K) member of the PG1159 class in a
pre-WD stage. Its optical spectrum is remarkable because it exhibits C IV lines
involving Rydberg states with principal quantum numbers up to n = 22. Its
rather low mass (0.48 +0.03/-0.02 Msun) is difficult to reconcile with the
common evolutionary scenario for PG1159 stars due to it being the outcome of a
(very) late He-shell flash. The same mass-problem faces a merger model of a
close He-sdO plus CO WD binary that predicts PG1159-like abundances. Perhaps RL
104 originates from a very late He-shell flash in a CO/He WD formed by a merger
of two low-mass He-WDs.
