  Low-mass M dwarfs represent the most common outcome of star formation, but
their complex emergent spectra hinder detailed studies of their composition and
initial formation. The measurement of isotopic ratios is a key tool that has
been used to unlock the formation of our Solar System, the Sun, and the nuclear
processes within more massive stars. We observed GJ 745AB, two M dwarfs
orbiting in a wide binary, with the IRTF/iSHELL spectrograph. Our spectroscopy
of CO in these stars at the 4.7 micron fundamental and 2.3 micron
first-overtone rovibrational bandheads reveals 12C16O, 13C16O, and 12C18O in
their photospheres. Since the stars are fully convective, the atomic
constituents of these isotopologues should be uniformly mixed throughout the
stars' interiors. We find that in these M dwarfs, both 12C/13C and 16O/18O
greatly exceed the Solar values. These measurements cannot be explained solely
by models of Galactic chemical evolution, but require that the stars formed
from an ISM significantly enriched by material ejected from an exploding
core-collape supernova. These isotopic measurements complement the elemental
abundances provided by large-scale spectroscopic surveys, and open a new window
onto studies of Galactic evolution, stellar populations, and individual
systems.
