  Mainstream presolar SiC grains from primitive meteorites show a clear
s-process signature in the isotopic composition of heavy trace elements. These
grains most likely condensed in the winds of a variety of AGB stars. However,
the non-solar and correlated Si and Ti isotopic compositions measured in these
grains are inconsistent with a pure s-signature. We present a possible solution
to this much-discussed problem by assuming a spread in the original composition
of parent AGB stars due to small chemical inhomogeneities in the interstellar
medium at the time of their birth. These inhomogeneities may naturally arise
from variations in the contributions of each nuclide to the interstellar medium
by the relevant stellar nucleosynthetic sites, SNII and different subtypes of
SNIa.
