Three "onion type" double carbon cages (consisting of an inner sp3-hybridized C60 buckminsterfullerene skeleton and an outer larger carbon cage with icosahedral symmetry, with covalent bonds between the carbon atoms of the 5-membered rings) are described and their strain energies are discussed : (i) when the outer cage is a C120 cage derived from buckminsterfullerene by replacing each bond between two six-membered rings by an acetylenic group, the strain is smallest ; (ii) when the same derivation concerns each bond between 5- and 6-membered rings leading to a C180 outer cage, the strain is largest ; (iii) when the outer cage is C180 with benzenoid rings, the strain is intermediate.
