Discovery and spectroscopic study of the massive Galactic cluster Mercer
  81
  During the last decade, hundreds of young massive cluster candidates have
been detected in the disk of the Milky Way. We investigate one of these
candidates, Mercer 81, which was discovered through a systematic search for
stellar overdensities, with follow-up NICMOS/HST infrared narrow-band
photometry to find emission-line stars and confirm it as a massive cluster.
Surprisingly, the brightest stars turned out to be a chance alignment of
foreground stars, while a real massive cluster was found among some fainter
stars in the field. From a first spectroscopic study of four emission-line
stars (ISAAC/VLT), it follows that Mercer 81 is a very massive young cluster,
placed at the far end of the Galactic bar. Additionally, in this work we
present some unpublished spectra from a follow-up observation program which
confirm that the cluster hosts several Nitrogen-rich Wolf-Rayet stars (WN) and
blue supergiants.
