The reaction sequences governing the reaction How in the rp process are important for the understanding of the energy generation and nucleosynthesis of heavy elements in hot and explosive stellar hydrogen burning. Of considerable interest are (p, n) reactions along the process path which lead to the formation of reaction cycles rather than to chains of proton capture processes and P decays. Previous direct attempts to measure the low-energy reaction cross sections for 'P(p, n) Si and Cl(p, a) S resulted only in upper limits for the strengths of possible low-energy resonances which may dominate the reaction rates. In this paper an indirect experimental approach is presented to study the structure of the low-energy unbound states in the compound nuclei S and Ar. The results allow a more accurate determination of the contributions of these low-energy levels in the (p, n) reaction channel.PACS number(s): 25.40.Ny, 25.40.Hs, 27.30.+t, 97.10.Cv