  The formation of sdBs is still puzzling, as is the chemical composition of
their atmospheres. While helium and other light elements are depleted relative
to solar values, heavy elements are highly enriched. Diffusion processes in the
hot, radiative atmosphere of these stars are the most likely explanation.
Although several attempts were made, it was not yet possible to model all the
observed features of sdB atmospheres. A setback of most prior studies was the
small sample size. We present a detailed abundance analysis of 68 sdBs. From
high resolution spectra obtained with the VLT/UVES instrument in the course of
the ESO Supernova Progenitor Survey (SPY) we measured elemental abundances of
up to 24 different ions per star. A general trend of enrichment was found with
increasing temperature for most of the heavier elements. The lighter elements
like carbon, oxygen and nitrogen are depleted irrespective of the temperature.
Although there is considerable scatter from one star to another, the general
abundance patterns in most sdBs are similar. An interplay between gravitational
settling, radiative levitation and weak winds is most likely responsible. About
6 % of the analysed stars show an enrichment in carbon and helium which cannot
be explained in the framework of diffusion alone. Nuclear processed material
must have been transported to the surface. The late hot-flasher scenario may
provide a possible explanation for this effect.
