  In two dimensional foams at equilibrium, triangular bubbles can be freely
exchanged with 3-fold stars --three edges ending at a central vertex. This
theorem is deduced here from Moukarzel's duality. Moreover, to probe the
method, a few related properties are established: under slow gas diffusion, T2
processes are continuous for triangles but not for other types of bubbles. In
general, the gas flow results in different configurations in the presence of a
triangle than in the presence of a star.
