  We report C, Si, and S isotope measurements on 34 presolar silicon carbide
grains of Type AB, characterized by 12C/13C < 10. Nitrogen, Mg-Al-, and
Ca-Ti-isotopic compositions were measured on a subset of these grains. Three
grains show large 32S excesses, a signature that has been previously observed
for grains from supernovae (SNe). Enrichments in 32S may be due to
contributions from the Si/S zone and the result of S molecule chemistry in
still unmixed SN ejecta or due to incorporation of radioactive 32Si from C-rich
explosive He shell ejecta. However, a SN origin remains unlikely for the three
AB grains considered here, because of missing evidence for 44Ti, relatively low
26Al/27Al ratios (a few times 10-3), and radiogenic 32S along with low 12C/13C
ratios. Instead, we show that born-again asymptotic giant branch (AGB) stars
that have undergone a very-late thermal pulse (VLTP), known to have low 12C/13C
ratios and enhanced abundances of the light s-process elements, can produce
32Si, which makes such stars attractive sources for AB grains with 32S
excesses. This lends support to the proposal that at least some AB grains
originate from born-again AGB stars, although uncertainties in the born-again
AGB star models and possible variations of initial S-isotopic compositions in
the parent stars of AB grains make it difficult to draw a definitive
conclusion.
