  We present here the application of a model for a mass circulation mechanism
in between the H-burning shell and the base of the convective envelope of low
mass AGB stars, aimed at studying the isotopic composition of those presolar
grains showing the most extreme levels of 18O depletion and high concentration
of 26Mg from the decay of 26Al. The mixing scheme we present is based on a
previously suggested magnetic-buoyancy process, already shown to account
adequately for the formation of the main neutron source for slow neutron
captures in AGB stars. We find that this scenario is also capable of
reproducing for the first time the extreme values of the 17O/16O, the 18O/16O,
and the 26Al/27Al isotopic ratios found in the mentioned oxide grains,
including the highest amounts of 26Al there measured.
