Context : Evolved low-mass stars ( 0.8 \leq M/M _ { \odot } \leq 2.5 ) of a wide range of metallicity bear signatures of a non-standard mixing event in their surface abundances of Li , C , and N , and in their ^ { 12 } C/ ^ { 13 } C ratio .
A Na overabundance has also been reported in some giants of open clusters but remains controversial .
The cause of the extra-mixing has been attributed to thermohaline convection that should take place after the RGB bump for low-mass stars and on the early-AGB for more massive objects .

Aims : To track the occurrence of this process over a wide mass range , we derive in a homogeneous way the abundances of C , N , O , and Na , as well as the ^ { 12 } C/ ^ { 13 } C ratio in a sample of 31 giants of 10 open clusters with turn-off masses from 1.7 to 3.1 M _ { \odot } .
The sample includes red giants , clump giants , and early-AGB stars .
We study the observational behavior of the abundances as well as the possible correlations between different elements and between the chemical abundances and stellar mass .

Methods : A model atmosphere analysis is conducted using high signal-to-noise ratio , high-resolution FEROS and EMMI spectra .
We derive atmospheric parameters using Fe i and Fe ii lines .
We calculate abundances for Na , C , N , and O , as well as the ^ { 12 } C/ ^ { 13 } C ratio using spectral synthesis .
For the elements Mg , Ca , Si , Sc , Ti , V , Cr , Co , and Ni , abundances are derived using equivalent widths .

Results : A group of first ascent red giants with M/M _ { \odot } \leq 2.5 exhibits lower [ N/C ] ratios than those measured in clump giants of the same mass range , suggesting an additional increase in the [ N/C ] ratio after the first dredge-up .
The sodium abundances corrected from NLTE are found to be about solar .
[ Na/Fe ] shows a slight increase of 0.10 dex as a function of stellar mass in the 1.8 to 3.2 M _ { \odot } range covered by our sample , in agreement with standard first dredge-up predictions .
Our results do not support previous claims of sodium overabundances as high as +0.60 dex .
An anti-correlation between ^ { 12 } C/ ^ { 13 } C and turn-off mass is identified and interpreted as being caused by a post-bump thermohaline mixing .
Moreover , we find low ^ { 12 } C/ ^ { 13 } C ratios in a few intermediate-mass early-AGB stars , confirming that an extra-mixing process also operates in stars that do not experienced the RGB bump .
In this case , the extra-mixing possibly acts on the early-AGB , in agreement with theoretical expectations for thermohaline mixing .

Conclusions :