We investigated by means of transmission electron microscopy (TEM) GaAs-AlGaAs core-shell nanowires (NWs) grown by Au-catalyzed metalorganic vapor phase epitaxy under As-rich vapor conditions. The structural analysis reveals that the NWs exhibit zincblende phase only and a very low density of twin defects along the NW trunk; a higher amount of twins around the NW axial (growth) direction occur instead within a tapered region closeby the NW tip. We correlate this finding to the Al-rich composition of the tapered region, as deduced by scanning-TEM (STEM) mass contrast analysis, this region being the effect of residual Au-assisted self-assembly of the material during shell overgrowth. In addition a few percent of NWs show branching during their growth, and we determine the crystal properties and defects of branched NWs. We propose that this phenomenon may be induced by Au nanoparticle instabilities …