  In this paper, we point out that a measurement of the charge radius of
Boron-8 provides indirect access to the S-factor for radiative proton capture
on Beryllium-7 at low energies. We use leading-order halo effective field
theory to explore this correlation and we give a relation between the charge
radius and the S-factor. Furthermore, we present important technical aspects
relevant to the renormalization of pointlike P-wave interactions in the
presence of a repulsive Coulomb interaction.
