  C and O isotopic ratios are reported for a sample of 46 Mira and SRa-type
variable AGB stars. Vibration-rotation 1st and 2nd overtone CO lines in 1.5 to
2.5 $\mu$m spectra were measured to derive isotopic ratios for 12C/13C,
16O/17O, and 16O/18O. Comparisons with previous measurements for individual
stars and with various samples of evolved stars are discussed. Models for solar
composition AGB stars of different initial masses are used to interpret our
results. We find that the majority of the M stars had main sequence masses < 2
Msun and have not experienced sizable third dredge-up episodes. The progenitors
of the four S-type stars in our sample are slightly more massive. Of the 6 C
stars in the sample three have clear evidence relating their origin to the
occurrence of the third dredge-up. Comparisons with O-rich presolar grains from
AGB stars that lived before the formation of the solar system reveal variations
in the interstellar medium chemical composition. The present generation of
low-mass AGB stars show a large spread of 16O/17O ratios, similar to that of
group 1 presolar grains and in agreement with theoretical expectations for the
composition of mass 1.2 to 2 Msun stars after the 1st dredge up. On the
contrary, the 16O/18O ratios of present-day LPVs are definitely smaller than
those of group 1 grains. This occurrence is most probably a consequence of the
the decrease with time of the 16O/18O ratio in the interstellar medium due to
the chemical evolution of the Milky Way. One star in our sample has an O
composition similar to that of group 2 presolar grains originating in an AGB
star undergoing extramixing. This occurrence may indicate that the extramixing
process is hampered at high metallicity or, equivalently, favored at low
metallicity. Similar to O-rich grains no star in our sample shows evidence of
HBB, expected for massive AGB stars.
