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It is becoming clear that `self--pollution' by the ejecta of massiveasymptotic giant branch stars has an important role in the early chemicalevolution of globular cluster stars, producing CNO abundance spreads which areobserved also at the surface of unevolved stars. Considering that the ejectawhich are CNO processed must also be helium enriched, we have modelled stellarevolution of globular cluster stars by taking into account this possible heliumenhancement with respect to the primordial value. We show that the differencesbetween the main evolutionary phases (main sequence, turn--off and red giants)are small enough that it would be very difficult to detect themobservationally. However, the difference in the evolving mass may play a rolein the morphology of the horizontal branch, and in particular in the formationof blue tails, in those globular clusters which show strong CNO abundancevariations, such as M13 and NGC 6752.

Helium variation due to self–pollution among Globular Cluster stars