Abstract : The 13C(alpha,n)16O reaction is considered the main neutron source for the s process in low mass asymptotic giant branch (AGB) stars. In the Gamow peak, the cross section sensitively depends on the 1/2+ subthreshold state of 17O (Ex=6.356 MeV). In this work, we determined the astrophysical S factor through an evaluation of the alpha spectroscopic factor and the corresponding asymptotic normalization factor (ANC) of the 6.356 MeV state using the transfer reaction 13C(7Li,t)17O at two different incident energies. Our result confirms that the contribution of the 1/2+ state is dominant at astrophysical energies. Our reaction rate at T=0.09 GK is slightly lower than the value adopted in the Nuclear Astrophysics Compilation of REaction rates (NACRE), but two times larger than the one obtained in a recent ANC measurement.