  (Context) When a galaxy acquires material from the outside, it is likely that
the resulting angular momentum of the accreted material is decoupled from that
of the pre-existing galaxy. The presence of stars counter-rotating with respect
to other stars and/or gas represents an extreme case of decoupling. (Aims)
NGC5719, an almost edge-on Sab galaxy with a prominent skewed dust lane, shows
a spectacular on-going interaction with its face-on Sbc companion NGC5713.
Observations of such interacting systems provide insight into the processes at
work in assembling and reshaping galaxies. (Methods) Studies were made of the
distribution and kinematics of neutral hydrogen in the NGC5719/13 galaxy pair
and the ionised gas and stellar kinematics along the major axis of NGC5719.
(Results) Two HI tidal bridges that loop around NGC5719 and connect to NGC5713,
and two HI tidal tails departing westward from NGC5713 were detected. There is
a correspondence between the HI condensations and the location of clumps of
young stars within and outside the disc of NGC5719. The low-mass satellite
PGC135857 at the tip of the northern tail was detected in HI, and is likely a
by-product of the interaction. The neutral and ionised hydrogen in the disc of
NGC5719 are counter-rotating with respect to the main stellar disc. The
counter-rotating stellar disc contains about 20% of the stars in the system,
and has the same radial extension as the main stellar disc. This is the first
interacting system in which a counter-rotating stellar disc has been detected.
(Conclusions) The data support a scenario where HI from the large reservoir
available in the galaxy's surroundings was accreted by NGC5719 onto a
retrograde orbit and subsequently fuelled the in-situ formation of the
counter-rotating stellar disc.
