  We present high resolution HST ultraviolet spectroscopy of NGC3242, the only
planetary nebula with a measured abundance of 3He. The Space Telescope Imaging
Spectrograph (STIS) has been used to observe the CIII multiplet near 1908 A.
The presence of 12C and 13C lines at these wavelengths allows a direct estimate
of the carbon isotopic ratio in the ionized gas of the nebula. We have detected
the 12C doublet and obtained an upper limit on the 13C 1909.6 A line, resulting
in a carbon isotopic ratio 12C/13C > 38, in agreement with standard stellar
models. The lack of the 13C line and the presence of the 3He line in the
spectrum indicate that the progenitor star did not undergo a phase of deep
mixing during the last stages of its evolution. The significance of this result
for studies of stellar nucleosynthesis and Galactic chemical evolution is
discussed.
