The behaviour of the \Delta \nu =2 CO bands around 2.3 \mu m was examined by comparing observed and synthetic spectra in stars in globular clusters of different metallicity .
Changes in the ^ { 12 } C/ ^ { 13 } C isotopic ratio and the carbon abundances were investigated in stars from 3500–4900 K in the galactic globular clusters M71 , M5 , M3 and M13 , covering the metallicity range from –0.7 to –1.6 .
We found relatively low carbon abundances that are not affected by the value of oxygen abundance .
For most giants the ^ { 12 } C/ ^ { 13 } C ratios determined are consistent with the equilibrium value for the CN cycle .
This suggests complete mixing on the ascent of the red giant branch , in contrast to the substantially higher values predicted across this range of parameters by the current generation of models .
We found some evidence for a larger dispersion of ^ { 12 } C/ ^ { 13 } C in giants of M71 of metallicity [ \mu ] = [ M/H ] = -0.7 in comparison with the giants of M3 , M5 and M13 , which are more metal deficient .
Finally , we show evidence for lower ^ { 12 } C/ ^ { 13 } C in giants of globular clusters with lower metallicities , as predicted by theory .