  We suggest that a singlet fermion $S$ exists beyond the standard see-saw
structure. It mixes with light neutrinos via interactions with the right-handed
neutrino components, so that $\n_e \to S$ conversion solves the solar neutrino
problem. Supersymmetry endowed with R-symmetry is shown to give a natural
framework for existence, mass scale and mixing ($\sin^22\th_{es} \sim
(0.1-1.5)\cdot 10^{-2}$) of such a fermion. Models with an approximate
horizontal symmetry are constructed, which embed the fermion $S$ and explain
simultaneously solar, atmospheric, hot dark matter problems as well as may
predict the oscillation $\bar{\n}_\m \to \bar{\n}_e$ in the region of
sensitivity of KARMEN and LSND experiments.
