On the peculiar red clump morphology in the open clusters NGC 752 and NGC 7789

The red clump stars in the open cluster NGC 752 present a peculiar distribu tion in the colour-magnitude diagram (CMD): the clump is observed to presen t a faint extension, slightly to the blue of the main concentration of clum p stars. We point out that a similar structure is present in the CMD of NGC 7789, and discuss their possible origins. This feature may be understood a s the result of having, at the same time, stars of low-mass which undergo t he helium-flash and those just massive enough for avoiding it. The ages of both clusters are compatible with this interpretation. Similar features can be produced in theoretical models which assume a non-n egligible mass spread for clump stars, of about 0.2 M-circle dot. However, one can probably exclude that the observed effect is due to the natural mas s range of core helium burning stars found in single isochrones, although p resent models do not present the level of detail necessary to completely ex plore this possibility. Also the possibility of a large age spread among cl uster stars can be refuted on observational grounds. We then suggest a few alternatives. This spread may be resulting either fro m star-to-star variations in the mass-loss rates during the RGB phase. Alte rnatively effects such as stellar rotation or convective core overshooting, could be causing a significant spread in the core mass at He-ignition for stars of similar mass. Finally, we point out that similar effects could als o help to understand the distribution of clump stars in the CMDs of the clu sters NGC 2660 and NGC 2204.