Berkeley 51, a young open cluster with four yellow supergiants
  The heavily obscured open cluster Berkeley~51 shows characteristics typical
of young massive clusters, even though the few previous studies have suggested
older ages. We combine optical ($UBV$) and 2MASS photometry of the cluster
field with multi-object and long-slit optical spectroscopy for a large sample
of stars. We apply classical photometric analysis techniques to determine the
reddening to the cluster, and then derive cluster parameters via isochrone
fitting. We find a large population of B-type stars, with a main sequence
turn-off at B3$\,$V, as well as a large number of supergiants with spectral
types ranging from F to M. We use intermediate resolution spectra of the
evolved cool stars to derive their stellar parameters and find an essentially
solar iron abundance. Under the plausible assumption that our photometry
reaches stars still close to the ZAMS, the cluster is located at
$d\approx5.5\:$kpc and has an age of $\sim60\:$Ma, though a slightly younger
and more distant cluster cannot be ruled out. Despite the apparent good fit of
isochrones, evolved stars seem to reside in positions of the CMD far away from
the locations where stellar tracks predict Helium burning to occur. Of
particular interest is the presence of four yellow supergiants, two on the
ascending branch and two others close to or inside the instability strip.
