  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.
