Context : 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 Ultraluminous Infrared Galaxies ( ULIRGs ) although they show clear signs of interactions .
Whether external star formation may take place in such objects in an open question .

Aims : 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 .
The likelihood of survival of these regions as TDGs is also evaluated .

Methods : The analysis is based on optical Integral Field Spectroscopy ( IFS ) and high angular resolution HST imaging .

Results : 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 1 \times 10 ^ { 10 } M _ { \odot } , with average sizes of \sim 750 pc .
In addition , the line ratios ( H ii region-like ) , metallicities ( 12 + \log \mathrm { ( O / H ) } \sim 8.6 ) and H \alpha equivalent widths ( 34 - 257 Å ) are typical of young bursts of star formation ( age \sim 5 - 8 Myr ) , and similar to those of the TDG candidates found in less luminous mergers and compact groups of galaxies .
The extinction corrected H \alpha luminosity of these young bursts leads to masses for the young stellar component of \sim 2 \times 10 ^ { 6 } -7 \times 10 ^ { 8 } M _ { \odot } .

The likelihood of survival of these regions as TDGs is discussed based on their structural and kinematic properties .
Particularly interesting is our finding that most of these systems follow the relation between effective radius and velocity dispersion found at lower ( globular clusters ) and higher ( Elliptical ) mass systems , which suggests they are stable against internal motions .
The stability against forces from the parent galaxy is studied on the bases of several criteria 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 based on all the utilised tracers .
Our best candidate , which satisfy all the utilized criteria , is located in the advanced merger IRAS 15250+3609 and presents a velocity field decoupled from the relatively distant parent galaxy .

Conclusions :