  We investigate whether the observed high number of carbon- and
nitrogen-enhanced extremely metal-poor stars could be explained by peculiar
evolutionary properties during the core He flash at the tip of the red giant
branch. For this purpose we compute a series of detailed stellar models
expanding upon our previous work; in particular, we investigate if during the
major He flash the penetration of the helium convective zone into the overlying
hydrogen-rich layers can produce carbon- and nitrogen-rich abundances in
agreement with current spectroscopic observations. The dependence of this
phenomenon on selected model input parameters, such as initial metallicity and
treatment of convection is examined in detail.
