New Constraints on Neutrino Oscillations in Vacuum as a Possible Solution of the Solar Neutrino Problem

Two-neutrino oscillations in vacuum are studied as a possible solution of the solar neutrino problem. New constraints on the parameter sn2, characterizing the mixing of the electron neutrino with another active or sterile neutrino, as well as on the mass--squared difference, dm2, of their massive neutrino components, are derived using the latest results from the four solar neutrino experiments. Oscillations into a sterile neutrino are ruled out at 99 % C.L. by the observed mean event rates even if one includes the uncertainties of the standard solar model predictions in the analysis.Comment: 10 pages + 3 figures attached as postscript files, IFP-480-UNC and Ref. SISSA 177/93/EP (Updated Version which takes into account the latest GALLEX results from 30 runs