  We conduct an intensive study of the rich, X-ray luminous galaxy cluster A773
at z=0.22 containing a diffuse radio halo to determine its dynamical status.
Our analysis is based on new spectroscopic data obtained at the TNG telescope
for 107 galaxies, 37 spectra recovered from the CFHT archive and new
photometric data obtained at the INT telescope. We use statistical tools to
select 100 cluster members (out to ~1.8 Mpc from the cluster centre), to
analyse the kinematics of cluster galaxies and to determine the cluster
structure. Our analysis is also performed by using X-ray data stored in the
Chandra archive. Results: The 2D distribution shows two significant peaks
separated by ~2 arcmin in the EW direction with the main western one closely
located at the position of the two dominant galaxies and the X-ray peak. The
velocity distribution of cluster galaxies shows two peaks at v~65000 and ~67500
km/s, corresponding to the velocities of the two dominant galaxies. The low
velocity structure has a high velocity dispersion sigma_v=800-1100 km/s and its
galaxies are centred on the western 2D peak. The high velocity structure has
intermediate velocity dispersion sigma_v~500 km/s and is characterized by a
complex 2D structure with a component centred on the western 2D peak and a
component centred on the eastern 2D peak, these components probably being two
independent groups.We estimate a cluster mass within 1 Mpc of 6--11 e^14 h_70
Msun. Our analysis of Chandra data shows the presence of two very close peaks
in the core and the elongation of the X-ray emission in the NEE--SWW direction.
Our results suggest we are looking at probably two groups in an advanced stage
of merging. In particular, the radio halo seems to be related to the merger of
the eastern group.
