Just-so oscillation: as just as MSW?

The long wavelength (just-so) neutrino oscillation is reconsidered as a solution to the solar neutrino problem, in the context of various solar models with different original fluxes of relevant components (boron, beryllium neutrinos etc.). In the light of the presently updated results of the four solar neutrino experiments, the data fit in this scenario substantially improves and becomes almost as good as in the MSW scenario. Surprising result of our analysis is that the best data fit is achieved when the oscillation occurs only between the two neutrino states: switching on the oscillation into third neutrino increases the χX2 value. Namely, by considering the vacuum oscillation scenario in the three-neutrino system (4 parameters), we find out that the minimum of χX2 is always achieved in the two parameter subspace in which actually only two neutrino states oscillate. The possible implications of this observation for the neutrino mass matrix textures are also discussed.