  The ${^3{\rm He}}(\alpha,\gamma){^7{\rm Be}}$ and ${^3{\rm
H}}(\alpha,\gamma){^7{\rm Li}}$ astrophysical $S$ factors are evaluated at low
collision energies (less than 2.5 MeV in the centre-of-mass frame) within the
no-core shell model with continuum approach using a renormalized chiral
nucleon-nucleon interaction.
