Context :

Aims : We present abundances for Ru and Hf , compare them to abundances of other heavy elements , and discuss the problems found in determining Ru and Hf abundances with laboratory gf -values in the spectra of barium stars .

Methods : We determined Ru and Hf abundances in a sample of giant and dwarf barium stars , by the spectral synthesis of two Ru i ( \lambda 4080.574 and \lambda 4757.856 ) and two Hf ii ( \lambda 4080.437 and \lambda 4093.155 ) transitions .
The stellar spectra were observed with FEROS/ESO , and the stellar atmospheric parameters lie in the range 4300 < T _ { eff } /K < 6500 , - 1.2 < [ Fe/H ] \leq 0 and 1.4 \leq \log g < 4.6 .

Results : The Hf ii \lambda 4080 and the Ru i \lambda 4758 observed transitions result in a unreasonably high solar abundance , given certain known uncertainties , when fitted with laboratory gf -values .
For these two transitions we determined empirical gf -values by fitting the observed line profiles of the spectra of the Sun and Arcturus .
For the sample stars , this procedure resulted in a good agreement of Ru and Hf abundances given by the two available lines .
The resulting Ru and Hf abundances were compared to those of Y , Nd , Sm and Eu .
In the solar system Ru , Sm and Eu are dominated by the r -process and Hf , Nd and Y by the s -process , and all of these elements are enhanced in barium stars since they lie inside the s -process path .
Ru abundances show large scatter when compared to other heavy elements , whereas Hf abundances show less scatter and closely follow the abundances of Sm and Nd , in good agreement with theoretical expectations .
We also suggest a possible , unexpected , correlation of Ru and Sm abundances .
The observed behaviour in abundances is probably due to variations in the ^ { 13 } C pocket efficiency in AGB stars , and , though masked by high uncertainties , hint at a more complex scenario than proposed by theory .

Conclusions :