  We report the Sr and Ba isotopic compositions of 18 presolar SiC grains of
types Y (11) and Z (7), rare types commonly argued to have formed in
lower-than-solar metallicity asymptotic giant branch (AGB) stars. We find that
the Y and Z grains show higher 88Sr/87Sr and more variable 138Ba/136Ba ratios
than mainstream (MS) grains. According to FRANEC Torino AGB models, the Si, Sr,
and Ba isotopic compositions of our Y and Z grains can be consistently
explained if the grains came from low mass AGB stars with 0.15 Zsun <= Z < 1.00
Zsun, in which the 13C neutron exposure for the slow neutron-capture process is
greatly reduced with respect to that required by MS grains for a 1.0 Z8 AGB
star. This scenario is in line with the previous finding based on Ti isotopes,
but it fails to explain the indistinguishable Mo isotopic compositions of MS,
Y, and Z grains.
