  Recently a new class of hot subluminous stars strongly enriched in C and O
have been discovered (CO-sdOs). These stars show abundances very similar to
those observed in PG1159 stars but at lower temperatures. Moreover, it has been
recently suggested that C and O enrichment might be the key ingredient driving
the pulsations in He-rich hot subdwarf stars (He-sdBVs). Here we argue that
these two types of rare stars can be explained by a variant of one of the main
channels forming hot subdwarf stars. The scenario involves the formation and
merging of a He-core white dwarf and a less massive CO-core white dwarf. We
have constructed a simple merger models and computed their subsequent
evolution. The merger products are in agreement with the surface parameters and
composition of CO-sdOs. In addition, we have performed simulations including
the effects of element diffusion and the excitation of pulsations. These
simulations show that less massive merger products can form stellar structures
that have surface parameters, abundances, and pulsation periods similar to
those displayed by He-sdBVs. We conclude that the proposed scenario, or some
variant of it, offers a very plausible explanation for the formation of
CO-sdOs, pulsating He-sdBs and low-luminosity PG1159 stars.
