  (ABRIDGED) Star-forming small galaxies made out of collisional debris have
been found in a variety of merging systems. So far only a few of them are known
in ULIRGs although they show clear signs of interactions. Whether external star
formation may take place in such objects is an open question. The aim of this
paper is to identify and characterise the physical and kinematic properties of
the external star forming regions in a sample of ULIRGs, including TDG
candidates, using optical IFS and high angular resolution HST imaging. We have
found that the presence of external star-forming regions is common with 12
objects being identified in 5 ULIRGs. These regions show a large range of
dynamical mass up to 1x10^{10} M_sun, with average sizes of ~750 pc. In
addition, the line ratios, metallicities and H\alpha equivalent widths are
typical of young bursts of star formation (age ~ 5-8 Myr), and similar to those
of other TDG candidates. Their extinction corrected H\alpha luminosities lead
to masses for the young stellar component of ~2x10^6 - 7x10^8 M_sun. The
likelihood of survival of these regions as TDGs is discussed based on their
structural and kinematic properties. Most of these systems follow the relation
between effective radius and velocity dispersion found for globular clusters
and Ellipticals, which suggests they are stable against internal motions. The
stability against forces from the parent galaxy have been studied and a
comparison of the data with the predictions of dynamical evolutionary models is
also performed. Five regions out of twelve show High-Medium or High likelihood
of survival. Our best candidate, which satisfy all the utilized criteria, is
located in the advanced merger IRAS15250+3609 and presents a velocity field
decoupled from the relatively distant parent galaxy.
