Abstract : In this work, we developed a general tool for spectroscopic studies of unbound nuclear states. Within this method, the spectroscopic information is obtained using resonant elastic scattering in inverse kinematics. The method applies to both, thin and thick target measurements. The measurements were performed at 0° with respect to the beam axis. We used a thin target for the 4He(15O,α)15O reaction, and a thick one for 1H(15O,p)15O. The second measurement allowed us to obtain, with high accuracy, the properties (energy, spin, parity and level width) of the first states of the unbound 16F. The extracted spectroscopic information was used to estimate the 15O(p,β+)16O reaction rate. The estimated reaction rate is compared to that of 15O(α,γ)19Ne. For the first time the importance of a low energy tail of resonance in the unbound nucleus was shown. The strong hindrance of the proton decay probability due to the influence of the coulomb barrier should be observed in the tail of resonance. Thus the probability of beta decay of 16F to 16O should be enhanced. Furthermore, we proposed an efficient way to populate the low energy tail of the resonance using a γ transition. In this context, 15O(p,γ)(β+)16O and 15O(p,γ)(p,γ)17Ne reactions were analyzed for the first time and compared to the 15O(α,γ)19Ne reaction rate.Consequences of the above findings on the nucleosynthesis in novae and X-ray bursts are discussed.