The consequences are explored of assuming that, in common envelope events involving mass transfer in close binaries, the energy required to eject the envelope from the system is comparable with the change in orbital energy of the system. Situations in which the primary fills its Roche lobe for the first time (1) after it has exhausted central hydrogen, but before it has ignited helium, and (2) after it has already developed an electron-degenerate carbon-oxygen core are studied in detail. The properties of a system consisting of a CO degenerate dwarf and a helium star, with angular momentum loss due to gravitational wave radiation maintaining the helium star in Roche lobe contact, are explored. Finally, the formation frequency of non-Roche lobe filling helium stars with companions of various types is examined to see whether they could account for some fraction of observed sdB and sdO stars.
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Icko et al. (1987) studied this question.