We study the cluster structure of $^{12}$C and $^{13}$C in the framework of the cluster shell model. Simple relations are derived for ratios of longitudinal form factors as well as transition probabilities. It is shown that the available experimental data for $^{12}$C and $^{13}$C can be well described by a triangular structure with ${\cal D}_{3h}$ and ${\cal D}'_{3h}$ symmetry, respectively.
Comment: 11 pages, 4 figures, 2 tables, Phys. Lett. B