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We report C, Si, and S isotope measurements on 34 presolar silicon carbidegrains of Type AB, characterized by 12C/13C < 10. Nitrogen, Mg-Al-, andCa-Ti-isotopic compositions were measured on a subset of these grains. Threegrains show large 32S excesses, a signature that has been previously observedfor grains from supernovae (SNe). Enrichments in 32S may be due tocontributions from the Si/S zone and the result of S molecule chemistry instill unmixed SN ejecta or due to incorporation of radioactive 32Si from C-richexplosive He shell ejecta. However, a SN origin remains unlikely for the threeAB grains considered here, because of missing evidence for 44Ti, relatively low26Al/27Al ratios (a few times 10-3), and radiogenic 32S along with low 12C/13Cratios. Instead, we show that born-again asymptotic giant branch (AGB) starsthat have undergone a very-late thermal pulse (VLTP), known to have low 12C/13Cratios and enhanced abundances of the light s-process elements, can produce32Si, which makes such stars attractive sources for AB grains with 32Sexcesses. This lends support to the proposal that at least some AB grainsoriginate from born-again AGB stars, although uncertainties in the born-againAGB star models and possible variations of initial S-isotopic compositions inthe parent stars of AB grains make it difficult to draw a definitiveconclusion.

Evidence for Radiogenic Sulfur-32 in Type AB Presolar Silicon Carbide Grains?