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 N-14(p,gamma)O-15 reaction is treated as an example. A similar expansion derived for neutron capture provides simple low-energy parametrizations of the C-12(n,gamma)C-13 cross sections.