Star clusters with dual red clumps
  A few star clusters in the Magellanic Clouds exhibit composite structures in
the red-clump region of their colour-magnitude diagrams. The most striking case
is NGC419 in the SMC, where the red clump is composed of a main blob as well as
a distinct secondary feature. This structure is demonstrated to be real and
corresponds to the simultaneous presence of stars which passed through electron
degeneracy after central-hydrogen exhaustion and those that did not. This rare
occurrence in a single cluster allows us to set stringent constraints on its
age and on the efficiency of convective-core overshooting during main-sequence
evolution. We present a more detailed analysis of NGC419, together with a first
look at other populous LMC clusters which are apparently in the same phase:
NGC1751, NGC1783, NGC1806, NGC1846, NGC1852 and NGC1917. We also compare these
Magellanic Cloud cases with their Galactic counterparts, NGC752 and NGC7789. We
emphasise the extraordinary potential of these clusters as absolute calibration
marks on the age scale of stellar populations.
