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Published online by Cambridge University Press: 12 April 2016
If we believe in the spectral analysis of the atmospheres of RCrB stars these objects should have a pure helium envelope slightly enriched in carbon (SEARLE 1961, DANZIGER 1965). However, they cannot be homogeneous helium stars since in that case they should be on the helium main sequence in the Hertzsprung-Russell-diagram, whereas they are found in the region of late supergiants. Their internal chemical structure must therefore be more complicated than that of a homogeneous helium star. From the viewpoint of stellar evolution the next degree of complexity would be to have a carbon-oxygen core.