  We present the results from the dynamical analysis of the cluster of galaxies
RX J0152-1357, which shows a complex structure in its X-ray emission, with two
major clumps in the central region, and a third clump in the Eastern region.
Our analysis is based on redshift data for 187 galaxies. We compute the
line-of-sight velocity dispersion of galaxies, sigma_V=1322^+74_-68 kms^-1,
which is significantly larger than what is expected in the case of a relaxed
cluster with an observed X-ray temperature of 5-6 keV. We find evidence that
this cluster is far from dynamical equilibrium, as shown by the non Gaussianity
of the velocity distribution, the presence of a velocity gradient and
significant substructure. Our analysis shows that the high value of sigma_V is
due to the complex structure of RX J0152-1357, i.e. to the presence of three
galaxy clumps of different mean velocity. Using optical data we detect a
low-velocity clump (with sigma_V=300-500 kms^-1) in the central SW region and a
high--velocity clump (with sigma_V sim 700 kms^-1) in the E region, well
corresponding to the SW and E peaks detected in the X-ray emission. The central
NE X-ray peak is associated to the main galaxy structure with a velocity which
is intermediate between those of the other two clumps and sigma_V sim 900
kms^-1. The mass of the whole system within 2 Mpc is estimated to lie in the
range (1.2-2.2)X10^{15} M_sun. Analytic calculations based on the two-body
model indicate that the system is most likely bound, currently undergoing
merging. In particular, we suggest that the SW clump is not a small group, but
rather the dense cluster-core of a massive cluster.
