We report on a search for heavy neutrinos (upsilon (4)) produced in the dec
ay D-s --> tau upsilon (4) at the SPS proton target followed by the decay u
psilon (4) --> upsilon (tau)e(+)e(-) in the NOMAD detector. Both decays are
expected to occur if upsilon (4) is a component of upsilon (tau). From the
analysis of the data collected during the 1996-1998 runs with 4.1 x 10(19)
protons on target, a single candidate event consistent with background exp
ectations was found. This allows to derive an upper limit on the mixing str
ength between the heavy neutrino and the tau neutrino in the upsilon (4) ma
ss range from 10 to 190 MeV. Windows between the SN1987a and Big Bang Nucle
osynthesis lower limits and our result are still open fur future experiment
al searches. The results obtained are used to constrain an interpretation o
f the time anomaly observed in the KARMEN1 detector. (C) 2001 Published by
Elsevier Science B.V.