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Recently a new class of hot subluminous stars strongly enriched in C and Ohave been discovered (CO-sdOs). These stars show abundances very similar tothose observed in PG1159 stars but at lower temperatures. Moreover, it has beenrecently suggested that C and O enrichment might be the key ingredient drivingthe pulsations in He-rich hot subdwarf stars (He-sdBVs). Here we argue thatthese two types of rare stars can be explained by a variant of one of the mainchannels forming hot subdwarf stars. The scenario involves the formation andmerging of a He-core white dwarf and a less massive CO-core white dwarf. Wehave constructed a simple merger models and computed their subsequentevolution. The merger products are in agreement with the surface parameters andcomposition of CO-sdOs. In addition, we have performed simulations includingthe effects of element diffusion and the excitation of pulsations. Thesesimulations show that less massive merger products can form stellar structuresthat have surface parameters, abundances, and pulsation periods similar tothose displayed by He-sdBVs. We conclude that the proposed scenario, or somevariant of it, offers a very plausible explanation for the formation ofCO-sdOs, pulsating He-sdBs and low-luminosity PG1159 stars.

An evolutionary channel for CO-rich and pulsating He-rich subdwarfs