Neutrino-Modulino Mixing

We suggest an existence of light singlet fermion, $S$, which interacts with observable matter only via 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 stemming from superstrings. The $\nu S$ mixing may be accompanied by observable $R$-parity breaking effects.