  We set up a formalism to calculate the charge form factors of two-neutron
halo nuclei with S-wave neutron-core interactions in the framework of the halo
effective field theory. The method is applied to some known and suspected halo
nuclei. In particular, we calculate the form factors and charge radii relative
to the core to leading order in the halo EFT and compare to experiments where
they are available. Moreover, we investigate the general dependence of the
charge radius on the core mass and the one- and two-neutron separation
energies.
