We propose to determine the abundances of Ne and Mg in  WC and WO
stars, which are the last visible stage in massive  star evolution
before supernova explosions. These abundances are an
essential test of the nuclear reactions chains in the Helium-burning
stage. In particular, the  reaction involving Ne and Mg, i.e
22Ne(alpha, gamma)25Mg, is the source of neutrons, which are then
captured on Fe-peak elements, thus producing s-elements.

The abundances of Ne and Mg are critical to check whether massive
stars really are the main sources of s-elements  with atomic mass
number  A <  100, as found in several recent  nucleosynthetic models
(Prantzos, Hashimoto and Nomoto, 1990; Meynet and Arnould,1992;
Baraffe, El Eid, Prantzos, 1992). Before it was thought that AGB stars
were the main source of s-elements, a fact which seems to be true only
for  s-elements with A > 100, which are quantitatively less abundant.

The Ne and Mg abundances in WC and WO stars can ONLY be determined
from spectroscopy of their Infrared fine structure lines in the SWS
waveband. To date such [Ne] lines have been observed only for
Gamma Velorum (WC8+O9I) - the derived Ne/He ratio being about twice
solar, rather than 30 times solar predicted in evolution models.
It is crucial to measure the Ne (and Mg) abundances in other (single)
WC stages, to the most extreme WO phase, to confront and test the
models. ISO provides a unique opportunity to do this and check the
relatively advanced nuclear evolution phases not analysed so far,
and test the newly-proposed site of s-elements with A < 100.