  The observation of the exotic quarkonium state Z_c(3900) by the BESIII and
Belle collaborations supports the concept of hadronic molecules. Charmonium
states interpreted as such molecules would be bound states of heavy particles
with small binding energies. This motivates their description using an
effective theory with contact interactions. In particular, we focus on the
electromagnetic structure of the charged state Z_c(3900). Using first
experimental results concerning spin and parity, we interpret it as an S-wave
molecule and calculate the form factors as well as charge and magnetic radii up
to next-to-leading order. We also present first numerical estimations of some
of these observables at leading order.
