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