  There is an apparent dichotomy between the metal-poor ([Fe/H] <= -2) yet
carbon-normal giants and their carbon-rich counterparts. The former undergo
significant depletion of carbon on the red giant branch after they have
undergone first dredge-up, whereas the latter do not appear to experience
significant depletion. We investigate this in the context that the extra mixing
occurs via the thermohaline instability that arises due to the burning of
helium-3. We present the evolution of [C/Fe], [N/Fe] and carbon-12/carbon-13
for three models: a carbon-normal metal-poor star, and two stars that have
accreted material from a 1.5 solar mass AGB companion, one having received 0.01
solar masses of material and the other having received 0.1 solar masses. We
find the behaviour of the carbon-normal metal-poor stars is well reproduced by
this mechanism. In addition, our models also show that the efficiency of
carbon-depletion is significantly reduced in carbon-rich stars. This
extra-mixing mechanism is able to reproduce the observed properties of both
carbon-normal and carbon-rich stars.
