  Precision data from cosmology suggest neutrinos stream freely and hence
interact very weakly around the epoch of recombination. We study this issue in
a simple framework where neutrinos recouple instantaneously and stop streaming
freely at a redshift z_i. The latest cosmological data imply z_i < 1500, the
exact constraint depending somewhat on the assumed prior on z_i. This bound
translates into a limit on the coupling strength between neutrinos and
majoron-like particles phi, implying tau > 1 x 10^10 s (m_2/50 meV)^3 for the
decay nu_2 -> nu_1+phi.
