Illustration representing the merger stages classification used in this work. I (first approach): The two galaxies are clearly separated but approaching to each other, following our Δv_rel condition. II (first contact): The two galaxies overlap with no strong perturbations. IIIa (pre-merger overlap): The two galaxies overlap and show strong perturbations. IIIb (pre-merger disturbed): The two galaxies are clearly separated but show strong perturbations. IIIc (pre-merger double-nucleus): There is only one very perturbed object clearly showing two nuclei. IVa (merger diffuse-Nncleus): There is one very perturbed object with a very diffuse nucleus. IVb (merger compact-nucleus): There is one very perturbed object with a very luminous and compact nucleus. V (old merger): There is one galaxy with no strong perturbations but with a disturbed central morphology.