We use halo effective field theory to analyze the universal features of proton halo nuclei bound due to a large S-wave scattering length. Our work provides a fully field-theoretical treatment of bound halo nuclei in the presence of a repulsive Coulomb interaction. With a Lagrangian built from effective core and valence-proton fields, we derive a leading-order expression for the charge form factor. Within the same framework we also calculate the radiative proton capture cross section. We present general results at leading order that can be applied to any one-proton halo system bound in a relative S wave. We illustrate the method by studying the excited 1/2 + state of fluorine 17, for which we give results for the charge radius and the astrophysical S factor
