Comparison of grain data to AGB stellar models. Three-isotope Zr plots of KFC1h presolar HD graphite grains. The four grains with positive δ⁹⁶Zr₉₄ values are indicated as black full circles, all the others as grey full circles. Two KFC1h grains with positive δ⁹⁶Zr₉₄ that were previously analyzed by Nicolussi et al. (1998) are shown as open squares. Isotopic ratios in the envelopes of various low-mass AGB stars at solar (Z = 0.014; Z is the mass fraction of elements heavier than He) and subsolar metallicities (Karakas et al. 2018) are shown as colored lines. Markers along these prediction lines indicate isotopic ratios when the stellar envelope is C-rich (C > O), which is the condition for the formation of graphite grains. The horizontal and vertical grey dashed lines represent solar ratios. The uncertainties are 1σ.