  We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19)
and Z (18), rare types commonly argued to have formed in lower-than-solar
metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and
Z grain data with data for mainstream grains from AGB stars of close-to-solar
metallicity demonstrates that the three types of grains have indistinguishable
Mo isotopic compositions. We show that the Mo isotope data can be used to
constrain the maximum stellar temperatures (TMAX) during thermal pulses in AGB
stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with
the grain data for Mo isotopes points to an origin from low-mass (~1.5-3 Msun)
rather than intermediate-mass (>3-~9 Msun) AGB stars. Because of the low
efficiency of 22Ne({\alpha},n)25Mg at the low TMAX values attained in low-mass
AGB stars, model calculations cannot explain the large 30Si excesses of Z
grains as arising from neutron capture, so these excesses remain a puzzle at
the moment.
