Continuous-wave and time-resolved optical spectroscopy is used to examine a variety of InGaN/GaN quantum-well and quantum-box samples. grown by molecular beam epitaxy. The results are analyzed in order to clarify the respective influences of electric fields and of carrier localizations on radiative recombinations. The coupling of electron-hole pairs with LO-phonons is also studied in detail. from careful analysis of the size-dependent intensities of LO-phonon replica. From our attempt of modelling the Huang-Rhys factor, S. for excitons in these systems, we conclude that the observed optical recombinations are rather those of electrons and holes separately localized on different potential fluctuations.