  Linear CoCu_nCo clusters on Cu(111) are fabricated by means of atomic
manipulation. They represent a two-site Kondo system with tunable interaction.
Scanning tunneling spectroscopy reveals oscillations of the Kondo temperature
T_K with the number n of Cu atoms for n>=3. Density functional calculations
show that the Ruderman-Kittel-Kasuya-Yosida interaction mediated by the Cu
chains causes the oscillations. Calculations find ferromagnetic and
antiferromagnetic interaction for n=1 and 2, respectively. Both interactions
lead to a decrease of T_K as experimentally observed.
