  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.
