  Recent observational results have demonstrated an increase in the surface Na
abundance that correlates with stellar mass for red giants between 2 and 3
Msun. This trend supports evolutionary mixing processes as the explanation for
Na overabundances seen in some red giants. In this same mass range, the surface
Al abundance was shown to be constant. Our main aim was to extend the
investigation of the Na and Al surface abundances to giants more massive than 3
Msun. We sought to establish accurately whether the Na abundances keep
increasing with stellar mass or a plateau is reached. In addition, we
investigated whether mixing can affect the surface abundance of Al in giants
more massive than 3 Msun. We obtained new high-resolution spectra of 20 giants
in the field of 10 open clusters; 17 of these stars were found to be members of
9 clusters. The giants have masses between 2.5 Msun and 5.6 Msun. Abundances of
C, N, and O were determined using spectrum synthesis. The abundances of Na and
Al were corrected for non-local thermodynamic equilibrium effects (non-LTE).
Moreover, to extend the mass range of our sample, we collected from the
literature high-quality C, N, O, and Na abundances of 32 Galactic Cepheids with
accurate masses in the range between 3 Msun and 14 Msun. The surface abundances
of C, N, O, Na, and Al were compared to predictions of stellar evolution models
with and without the inclusion of rotation-induced mixing. The surface
abundances of most giants and Cepheids of the sample can be explained by models
without rotation. For giants above ~ 2.5 Msun, the Na abundances reach a
plateau level of about [Na/Fe] ~ 0.20-0.25 dex (in non-LTE). Our results
support previous works that found models with rotation to overestimate the
mixing effects in intermediate-mass stars. [abridged]

Solar and Stellar Astrophysics