  We suggest the existence of a light singlet fermion, $S$, which interacts
with observable matter only via a Planck mass suppressed interaction: $\sim
m_{3/2}/M_P$, where $m_{3/2}$ is the supergravity gravitino mass. If the mass
of the singlet equals $\sim m_{3/2}^2/M_P$, then $\nu_e \to S$ resonance
conversion solves the solar neutrino problem or leads to observable effects.
The $\nu S$-mixing changes supernova neutrino fluxes and has an impact on the
primordial nucleosynthesis. The singlet $S$ can originate as the supersymmetric
partner of the moduli fields in supergravity or low energy effective theory
steaming from superstrings. The $\nu S$-mixing may be accompanied by observable
R-parity breaking effects.
