CLOSING THE WINDOWS ON MEV TAN NEUTRINOS

We analyze various constraints on the ''visible'' decay modes of a mas sive tau neutrino, nu(tau) --> nu'gamma and nu tau --> nu'e(+)e(-), wh ere nu' is a light neutrino. We point out that the BEBC experiment imp lies a new upper limit on the v, transition magnetic moment, mu(tran) less than or equal to 1.1 x 10(-9) (MeV/m(nu tau))(2) mu(B). Combining BEBC constraints with constraints arising from supernova observations and primordial nucleosynthesis calculations, we show that these ''vis ible'' modes cannot be the dominant decay modes of the MeV tau neutrin o.