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tokens: channels, thus the results are extremely sensitive to the cross-section of the 27 Al( p , γ) 28 Si reaction: the comparison with the Al measured in E stars seems to require slightly lower rates, but the differences are not sufficiently significant to require a supplementary analysis on this argument. Sodium is the most delicate species entering this discussion, because the surface abundance reflects the equilibria between the production and the destruction mechanisms (see Section 4.3 ), both very sensitive to the temperature, thus exposed to significant variation during the TP phase. This explains the TP evolution of the surface sodium, shown in the right-hand panel of Fig. 3 . As reported in Table 1 , the average sodium of the ejecta, [ Na/Fe ] ∼ 0.5, is similar to the quantity observed in E stars. Despite the vigorous destruction of sodium in the advanced TP phases, the ejecta are enriched in sodium (by a factor ∼3), because during the initial part of the evolution the sodium in the envelope reaches abundances 10–20 times higher than in the matter from which the star formed; this can be clearly seen in the right-hand panel of Fig. 3 . Such a large increase in the surface sodium is partly due to the accumulation of 22 Ne during the SDU (see discussion in Section 4.2 ); an additional motivation is that the sodium enrichment of the envelope takes place during the initial TP phases, when dust production is at the maximum efficiency (region labelled as Mg in the right-hand panel of Fig. 3 ): this favours the release of large quantities of sodium-rich gas into the interstellar medium. 5.3 Can we obtain oxygen-free ejecta? Oxygen deserves a separate discussion. The SAGB yields reported in Table 1 have [ O/Fe ] ∼ −0.2, ∼4 times smaller than FG stars. On the other hand, Fig. 1 above and fig. 3 in Carretta et al. ( 2018 ) show that some stars belonging to group E have extremely low surface oxygen, up to 20 times lower than observed in FG stars; furthermore, some of these measurements are upper limits. We did several tests, by varying the HBB modelling and the description of mass-loss: our conclusion is the impossibility of producing SAGB ejecta with such low oxygen contents. The only possibility is that the stars suffer extremely low mass-loss during the initial TP phases, until most of the oxygen in the envelope is consumed, via p-capture. The rate of mass-loss required to accomplish this task should be below yr −1 , far too small for this high luminosity, expanded stars. In this case, the overall agreement between the yields of SAGB stars and the chemistry of E stars would be substantially worse, because the gas ejected would be practically sodium free and would show magnesium depletion and silicon enrichment far in excess of the quantities observed. On the other hand, it is not possible to reproduce all the oxygen abundances of E stars at the same time, given the large spread observed, amounting to almost 1 dex. Oxygen is very peculiar on