We describe an approach to synthesize colloidal nanocrystal heterodimers composed of CoPt3 and Au. The growth is based on the nucleation of gold domains on preformed CoPt3 nanocrystals. It is a highly versatile methodology which allows us to tune independently the size of the two domains in each dimer by varying several reaction parameters. The statistical analysis of the distribution of the domain sizes in the dimers and the compositional mapping achieved by dark field imaging and energy dispersive spectroscopy confirm that the two domains in each dimer are indeed made of CoPt3 and Au, respectively. Structural characterization by high-resolution transmission electron microscopy shows that the two domains, both having cubic fcc Bravais lattice, can share a common {111}, {100}, or {110} facet, depending on the size of the initial CoPt3 seeds. The magnetization measurements evidence a ferromagnetic …