We point out a simple relation between the nuclear vertex constant (NVC) and the overall normalization of the astrophysical $S$ factor. Using predicted values of the NVC for the virtual decay of ${}_{}{}^8{}_{}{}^{}\mathrm{B}\to {}_{}{}^7{}_{}{}^{}\mathrm{Be}+p$ we find $S_{17}\mathrm{}\left(0\right)\mathrm{}\approx 17.6$ eVb for ${}_{}{}^7{}_{}{}^{}\mathrm{Be}{(p,\gamma )}^8\mathrm{B}$ reactions, consistent with the low values extrapolated from direct capture measurements by Filippone et al. and by Vaughn et al. New possibilities, using proton transfer reactions, to measure the astrophysical $S$ factor indirectly are proposed.

• Received 10 June 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.2027