Behaviour of radiative-capture reactions near zero energy

The Taylor expansion of the astrophysical S factor for radiative-capture reactions between charged particles is calculated in a direct way near zero energy in the potential and R matrix models. The 14N(p,γ)15O reaction is treated as an example. A similar expansion derived for neutron capture provides simple low-energy parametrizations of the 12C(n,γ)13C cross sections.