Isotopic abundance ratios ^ { 24 } Mg : ^ { 25 } Mg : ^ { 26 } Mg are derived for 20 stars from high-resolution spectra of the MgH A-X 0-0 band at 5140Å .
With the exception of the weak G-band giant HR 1299 , the stars are dwarfs that sample the metallicity range -1.8 < [ Fe/H ] < 0.0 .
The abundance of ^ { 25 } Mg and ^ { 26 } Mg relative to the dominant isotope ^ { 24 } Mg decreases with decreasing [ Fe/H ] in fair accord with predictions from a recent model of galactic chemical evolution in which the Mg isotopes are synthesised by massive stars .
Several stars appear especially enriched in the heavier Mg isotopes suggesting contamination by material from the envelopes of intermediate-mass AGB stars .