  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. We
determined Ru and Hf abundances in a sample of giant and dwarf barium stars, by
the spectral synthesis of two RuI (4080.574A and 4757.856A) and two HfII
(4080.437A and 4093.155A) transitions. The stellar spectra were observed with
FEROS/ESO, and the stellar atmospheric parameters lie in the range 4300 <
Teff/K < 6500, -1.2 < [Fe/H] <= 0 and 1.4 <= log g < 4.6. The HfII 4080A and
the RuI 4758A 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 13C pocket
efficiency in AGB stars, and, though masked by high uncertainties, hint at a
more complex scenario than proposed by theory.
