Evolution of the surface ¹²C/¹³C value as a function of stellar luminosity for the 1.25   M_⊙ models including thermohaline instability and rotation-induced mixing (for initial rotation velocities of 50, 80, and 110 km s^-1 shown as solid red, dashed green, and dotted blue lines respectively). The non rotating case is also shown (black solid line). Observations along the evolutionary sequence of the open cluster M 67 are from Gilroy & Brown (1991). The triangle is for a subgiant star for which only a lower value could be obtained, while black squares and red circles correspond respectively to RGB and clump stars.