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 $��_e \to S$ resonance conversion solves the solar neutrino problem or leads to observable effects. The $��S$-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 $��S$-mixing may be accompanied by observable R-parity breaking effects.

Text compressed to 4 pages and references added. Version to appear in Phys. Rev. Lett