  The oldest stars in the universe retain to a great extent detailed
information on the chemical composition of the interstellar medium at the time
of their birth. Hence the earliest phases of Galactic chemical evolution and
nucleosynthesis in the early universe can be investigated by means of the old
metal-poor stars. For the majority of extremely metal-poor (EMP) stars the
element ratios follow a normal pattern, but 10-15% of the stars are enhanced in
heavy neutron-capture (r- or s-processes) elements by large factors, and about
20% are strongly enriched in carbon. The enhancement of some elements could be
the result of highly non-spherical supernova and inefficient mixing in the
early interstellar medium. Alternative, they could be due to mass transfer from
a former AGB or supernova binary companion that has now evolved to a white
dwarf or neutron star. If the latter explanation is true we can detect these
stars as long-period binaries. Radial velocity monitoring over a period of ~5
years of a sample of EMP stars with either r-process element and/or carbon
enhancement is presented. The results indicate that pure r-process and carbon
enhancements are not results of mass transfer from a binary companion.
