The contributions of various mechanisms to the production of the final nucleus C-12 in the reaction C-13 He-3, alpha C-12 are estimated. These are neutron stripping and the transfer of the heavy cluster Be-9 in the pole approximation or via a sequential transfer of virtual Be-8 and a neutron. It is shown that the sequential mechanism of heavy-cluster transfer must be taken into account in order to describe correctly experimental data over the whole angular range.