  We present low-resolution spectroscopy of the ionized gas in a sample of
optical knots located along the tidal features of 14 interacting galaxies and
previously selected as candidates of Tidal Dwarf Galaxies (TDGs). From redshift
measurements, we are able to confirm their physical association with the
interacting system in almost all cases. For most knots, the oxygen abundance
does not depend on the blue luminosity. The average, 12+log(O/H)=8.34+-0.20, is
typical of TDGs and comparable to that measured in the outer stellar disk of
spirals from which they were formed. A few knots showing low metallicities are
probably pre-existing low-mass companions. The estimated Ha luminosity of the
TDG candidates is higher than the one of typical individual HII regions in
spiral disks and comparable to the global Ha luminosity of dwarf galaxies. We
find several instances of velocity gradients with amplitudes apparently larger
than 100 km/s in the ionized gas in the tidal knots and discuss various
possible origins for the large velocity amplitudes. While we can exclude tidal
streaming motions and outflows, we cannot rule out projection effects with the
current resolution. The velocity gradients could be indicative of the internal
kinematics characteristic of self-gravitating objects. Higher resolution
spectra are required to confirm whether the tidal knots in our sample have
already acquired their dynamical independence and are therefore genuine Tidal
Dwarf Galaxies.
