  We report on a search for heavy neutrinos ($\nus$) produced in the decay
$D_s\to \tau \nus$ at the SPS proton target followed by the decay $\nudecay$ in
the NOMAD detector. Both decays are expected to occur if $\nus$ is a component
of $\nu_{\tau}$.\
  From the analysis of the data collected during the 1996-1998 runs with
$4.1\times10^{19}$ protons on target, a single candidate event consistent with
background expectations was found. This allows to derive an upper limit on the
mixing strength between the heavy neutrino and the tau neutrino in the $\nus$
mass range from 10 to 190 $\rm MeV$. Windows between the SN1987a and Big Bang
Nucleosynthesis lower limits and our result are still open for future
experimental searches. The results obtained are used to constrain an
interpretation of the time anomaly observed in the KARMEN1 detector.\
