Be→ α+ 3 He are calculated in an α-cluster model that employs a potential involving
forbidden states. The results are compared with the values extracted from the measured and
calculated astrophysical S factors of the processes t (α, γ) 7 Li and 3 He (α, γ) 7 Be at
extremely low energies. Potentials of αt and α 3 He interaction that closely reproduce
nuclear vertex constants extracted from experimental data are found