[F/Ba] vs. [Ba/Fe] in the sample stars with [Fe/H] ≤ −1.0. The squares show extragalactic carbon stars, and the pentagons represent extrinsic CH/Ba stars. The lines are theoretical predictions for 1.5 M_⊙ TP-AGB stars with low metallicity, assuming different mechanisms for the formation of the ¹³C pocket (see text for details). At these low metallicities, theoretical AGB models predict that the star becomes C-rich from first TDU episodes. Data points and theoretical lines are color-coded by [Fe/H]. Typical error bars are indicated.