  Limits at 90 % c.l. on the square of the mixing strength |U_tau4|^2 between
nu_tau and a mostly isosinglet heavy neutrino with mass in the range 10-290
MeV/c^2 are reported. The results were derived using the negative result of a
search for neutral particles decaying into two electrons conducted by the CHARM
collaboration in a neutrino beam dump experiment. Upper limits ~10^-4 were
obtained for neutrino masses larger than 160 MeV/c^2.
