and Na as well as Mg and Al. Recently, variations in the abundance of fluorine have been discovered in giants in M 4 where the amplitude of the dispersion exceeds that of O [@smith05].

It is generally assumed that the light element variations arise from proton-capture reactions (CNO-cycle, Ne-Na chain, and Mg-Al chain), though the specific nucleosynthetic site(s) remain elusive. One possibility for the origin of the star-to-star abundance variations is deep-mixing and internal nucleosynthesis within the observed stars. Evidence for this “evolutionary scenario” include C and N abundances [@ss91] that vary with location on the red giant branch (RGB). Extensive mixing down to very hot layers is necessary to change the surface composition of Na, Mg, and Al. Such mixing is not predicted by standard models and the proposed mechanisms include meridional circulation [@sm79], turbulent diffusion [@charbonnel95], and hydrogen-burning shell flashes [@fujimoto99; @aikawa01; @aikawa04]. An alternative possibility is pollution from intermediate-mass asymptotic giant branch stars (IM-AGBs), first suggested by @cottrell81 to explain the Na and Al enhancements observed in CN strong stars in NGC 6752. In IM-AGBs, hydrogen-burning at the base of the convective envelope, so-called hot bottom burning (HBB), can produce the observed C to Al abundance patterns. 