The observations of carbon isotopic ratios in evolved stars suggest that non standard mixing is acting in low mass stars as they are ascending the red giant branch. We propose a simple consistent mechanism, based on the most recent developments in the description of rotation-induced mixing by Zahn (1992), which simultaneously accounts for the low $^{ 12}$C/$^{ 13}$C ratios in globular cluster and field Pop II giants and for the lithium abundances in metal-poor giant stars. It also leads to the destruction of $^3$He produced on the main sequence in low mass stars. This should both naturally account for the recent measurements of $^3$He/H in galactic HII regions and allow for high values of $^3$He observed in some planetary nebulae.
Categories: astro-ph