  We investigate the phase diagram and the critical properties of the adsorbate
system sulphur/ruthenium(0001) in the coverage region $\frac{1}{4} < \Theta <
\frac{1}{3}$ by means of Monte-Carlo simulations of a simple lattice gas model
on a triangular lattice. The model contains only repulsive nearest and
next-nearest neighbor interactions. Combining results obtained by using both
Glauber and Kawasaki kinetics in the simulations we identify two tricritical
points, three coexistence regions and a eutectic point in the phase diagram of
the system. Our results agree with the findings of recent experimental work on
S/Ru(0001). Furthermore we are able to add certain details to the
experimentally observed phase diagram.
