The puzzle of the synthesis of the rare nuclide 138 La

The calculations of the p -process in the O/Ne layers of Type II supernovae are quite successful in reproducing the solar system content of p-nuclides. They predict, however, a signicant underproduction of the rare odd-odd nuclide 138 La. A model for the explosion of a 25 M star with solar metallicity is used to suggest that e-captures on 138 Ba may well be its most ecient production mechanism. The responsibility of an inadequate prediction of the 138 La and 139 La photodisintegration rates in the too low production of 138 La is also examined quantitatively. A detailed discussion of the theoretical uncertainties in these rates suggest that the required rate changes are probably too high to be fully plausible. Their measurement would be most welcome. They would help disentangling the relative contributions of thermonuclear and neutrino processes to the 138 La production.