We have assembled accurate abundance data for Li , C , and N as well as the ^ { 12 } C / ^ { 13 } C ratio for five field giants with [ Fe/H ] \simeq - 0.6 including Arcturus and two stars in the globular cluster 47 Tuc .
Using their very precise M _ { bol } values obtained from HIPPARCOS parallaxes , we can place them into an evolutionary sequence .
The sequence shows that the ^ { 12 } C / ^ { 13 } C ratios drops from \sim 20 to near 7 between M _ { bol } = +1 and +0.5 , while Li disappears .
At the same time the ^ { 12 } C / ^ { 14 } N ratio diminishes by 0.2 to 0.4 dex .
The two stars in 47 Tuc with Mbol near -2.0 show even lower ^ { 12 } C / ^ { 14 } N ratios by 0.4 dex indicating further mixing as they evolved to the top of the red giant branch .

These observations confirm the existence of an extra-mixing process that becomes efficient on the red giant branch only when the low-mass stars reach the so-called luminosity function bump .
We use the values of the carbon isotopic ratio observed in our sample to get constraints on the \mu -barriers that may shield the central regions of a star from extra-mixing .
We show that the same value of the critical gradient of molecular weight leads to ^ { 12 } C / ^ { 13 } C ratios observed at different metallicities .
This “ observational critical \mu -gradient ” is in very good agreement with the one which is expected to stabilize meridional circulation .
This result provides strong clues on the nature of the extra-mixing which occurs on the RGB , and indicates that it is related to rotation .