  Primitive meteorites contain small amounts of presolar minerals that formed
in the winds of evolved stars or in the ejecta of stellar explosions. Silicon
carbide is the best studied presolar mineral. Based on its isotopic
compositions it was divided into distinct populations that have different
origins: Most abundant are the mainstream grains which are believed to come
from 1.5-3 Msun AGB stars of roughly solar metallicitiy. The rare Y and Z
grains are likely to come from 1.5-3 Msun AGB stars as well, but with subsolar
metallicities (0.3-0.5x solar). Here we report on C and Si isotope and trace
element (Zr, Ba) studies of individual, submicrometer-sized SiC grains. The
most striking results are: (1) Zr and Ba concentrations are higher in Y and Z
grains than in mainstream grains, with enrichments relative to Si and solar of
up to 70x (Zr) and 170x (Ba), respectively. (2) For the Y and Z grains there is
a positive correlation between Ba concentrations and amount of s-process Si.
This correlation is well explained by predictions for 2-3 Msun AGB stars with
metallicities of 0.3-0.5x solar. This confirms low-metallicity stars as most
likely stellar sources for the Y and Z grains.
