We present an analysis of a high resolution ( R \sim 50 , 000 ) optical spectrum of the central region of the proto-planetary nebula CRL 2688 .
This object is thought to have recently moved off the AGB , and display abundance patterns of CNO and heavy elements that can provide us with important clues to understand the nucleosynthesis , dredge-up and mixing experienced by the envelope of the central star during its AGB stage of evolution .
The analysis of the molecular features , presumably originated from the circumstellar matter provides further constraints on the chemistry and velocity of the expanding shell , expelled as a consequence of the strong mass loss experienced by the central star .

We confirm that the central star shows a spectrum typical of an F-type supergiant with T _ { eff } = 7250 \pm 400 K , \log g = 0.5 and [ Fe/H ] = -0.3 \pm 0.1 dex .
We find that the abundance pattern of this object is characterized by enhancements of Carbon ( [ C/Fe ] = 0.6 \pm 0.1 ) , Nitrogen ( [ N/Fe ] = 1.0 \pm 0.3 ) and Na ( [ Na/Fe ] = 0.7 \pm 0.1 ) , similar to other previously known carbon-rich post-AGB stars .
Yttrium is also enhanced while the [ Ba/Y ] ratio is very low ( -1.0 ) , indicating that only the light s-process elements are enhanced .
The Zinc abundance is found to be normal , [ Zn/Fe ] = 0.0 \pm 0.3 , suggesting that there is no depletion of refractory elements .
The H \alpha , Na i and K i resonance lines show prominent emission components , whose helio-centric radial velocities are offsetted by -41 \pm 3 km s ^ { -1 } relative to the photospheric metal-absorption lines .
The molecular features of C _ { 2 } and CN also show emission components , whose velocities are consistent with the emission components of the H \alpha , Na i and K i lines .
On the other hand , their absorption components are more highly blue shifted than the corresponding emission components , which suggests that the regions where the emission and absorption components arise are expanding at different velocities .