r-process nucleosynthesis in a double neutron-star merger: the case of GW170817

Double neutron star (NS-NS) mergers have been predicted as powerful gravitational waves and electromagnetic sources and are among the main targets of current gravitational wave detectors. These sources are also expected to be important astrophysical sites for rapid neutron capture (r-process) nucleosynthesis at the origin of the ''Kilonova'' (or ''macronova'') emission. So far only one NS-NS sytem (GW 170817) has been discovered with the current ground based gravitational wave laser interferometers. In this proceeding I will review the main observational properties and the still ongoing interpretation of the GW 170817 optical/NIR counterpart, AT2017gfo, recognized as the first convincing evidence of the existence of kilonovae and of the r-process nucleosynthesis from NS-NS system.