"Secret" neutrino interactions

We review the information about a potentially strong non-standard four-neutrino interaction that can be obtained from available experimental data. By using LEP results and nucleosynthesis data we find that a contact four-fermion neutrino interaction that involve only left-handed neutrinos or both left-handed and right-handed neutrinos cannot be stronger than the standard weak interactions. A much stronger interaction involving only right-handed neutrinos is still allowed.