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Diffusion of elements in the atmosphere and envelope of a star candrastically alter its surface composition, leading to extreme chemicalpeculiarities. We consider the case of hot subdwarfs, where surface heliumabundances range from practically zero to almost 100 percent. Since hotsubdwarfs can form via a number of different evolution channels, a key questionconcerns how the formation mechanism is connected to the present surfacechemistry. A sequence of extreme horizontal branch star models was generated byproducing post-common envelope stars from red giants. Evolution was computedwith MESA from envelope ejection up to core-helium ignition. Surface abundanceswere calculated at the zero-age horizontal branch for models with and withoutdiffusion. A number of simulations also included radiative levitation. The goalwas to study surface chemistry during evolution from cool giant to hot subdwarfand determine when the characteristic subdwarf surface is established. Onlystars leaving the giant branch close to core-helium ignition becomehydrogen-rich subdwarfs at the zero-age horizontal branch. Diffusion, includingradiative levitation, depletes the initial surface helium in all cases. Allsubdwarf models rapidly become more depleted than observations allow. Surfaceabundances of other elements follow observed trends in general, but not indetail. Additional physics is required.

The effects of diffusion in hot subdwarf progenitors from the common envelope channel