  We have determined the dynamical mass of the most luminous stellar cluster
known to date, i.e. object W3 in the merger remnant galaxy NGC 7252. The
dynamical mass is estimated from the velocity dispersion measured with the
high-resolution spectrograph UVES on VLT. Our result is the astonishingly high
velocity dispersion of sigma=45 +- 5 km/s. Combined with the large cluster size
R_eff=17.5 +-1.8 pc, this translates into a dynamical virial mass for W3 of 8
+- 2 x 10^7 Msun. This mass is in excellent agreement with the value 7.2 x 10^7
Msun we previously estimated from the cluster luminosity M_V=-16.2 by means of
stellar M/L ratios predicted by Simple Stellar Population models (with a
Salpeter IMF) and confirms the heavy-weight nature of this object. This results
points out that the NGC 7252-type of mergers are able to form stellar systems
with masses up to ~ 10^8 Msun. We find that W3, when evolved to ~ 10 Gyr, lies
far from the typical Milky Way globular clusters, but appears to be also
separated from omegaCen in the Milky Way and G1 in M31, the most massive old
stellar clusters of the Local Group, because it is too extended for a given
mass, and from dwarf elliptical galaxies because it is much more compact for
its mass. Instead the aged W3 is amazingly close to the compact objects named
ultracompact dwarf galaxies (UCDGs) found in the Fornax cluster (Hilker et al.
1999; Drinkwater et al. 2000), and to a miniature version of the compact
elliptical M32. These objects start populating a previously deserted region of
the fundamental plane.
