Ninety-two refractory oxide grains (primarily Al2O3) with highly unusual O-isotopic ratios have been found in acid-resistant residues of five primitive meteorites. Thirty-five of these also have large excesses of 26Mg, attributable to the in situ decay of radioactive 26Al. The extreme ranges of isotopic compositions of the grains indicate that they are unprocessed stellar condensates. The grains have been divided into four groups. Group 1 grains have 17O excesses and moderate 18O depletions, relative to solar, and most likely formed around red giants and asymptotic giant branch (AGB) stars. However, many individual stars with different masses and initial compositions are required to explain the range of O-isotopic ratios and inferred 26Al/27Al ratios observed in the grains. Group 3 grains, which have 17O and 18O depletions, probably originated in O-rich red giants of very low mass (M≲1.4M) and low metallicity. The Group 3 grains’ compositions are probably strongly influenced by the chemical evolution of the Galaxy; they also provide a new method of determining the age of our Galaxy. Group 2 grains have large 18O depletions, 17O enrichments and high inferred 26Al/27Al ratios; they probably formed in low-mass AGB stars in which extra mixing (“cool bottom processing”) occurred. The four Group 4 grains have 18O enrichments. Possible explanations for these excesses include dredge-up of this isotope in early thermal pulses in AGB stars or an origin in low-mass red giants of unusually high metallicity. One grain, T54, is extremely enriched in 17O and depleted in 18O, and may have formed in an AGB star undergoing hot-bottom-burning. Presolar oxides are underabundant in meteorites, relative to presolar SiC, perhaps because Al condenses more readily into silicates than into refractory oxides or because presolar Al2O3 has a finer grain size distribution. No presolar oxide grains from supernovae have been identified, despite expectations that they should be present.

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