  It has been noted that, in classical extremely metal-poor (EMP) stars, the
abundance ratio of Sr and Ba, is always higher than [Sr/Ba] = -0.5, the value
of the solar r-only process; however, a handful of EMP stars have recently been
found with a very low Sr/Ba ratio. We try to understand the origin of this
anomaly by comparing the abundance pattern of the elements in these stars and
in the classical EMP stars. Four stars with very low Sr/Ba ratios were observed
and analyzed within LTE approximation through 1D (hydrostatic) model
atmosphere, providing homogeneous abundances of nine neutron-capture elements.
In CS 22950-173, the only turnoff star of the sample, the Sr/Ba ratio is, in
fact, found to be higher than the r-only solar ratio, so the star is discarded.
The remaining stars (CS 29493-090, CS 30322-023, HE 305-4520) are cool evolved
giants. They do not present a clear carbon enrichment. The abundance patterns
of the neutron-capture elements in the three stars are strikingly similar to a
theoretical s-process pattern. This pattern could at first be attributed to
pollution by a nearby AGB, but none of the stars presents a clear variation in
the radial velocity indicating the presence of a companion. The stellar
parameters seem to exclude any internal pollution in a TP-AGB phase for at
least two of these stars. The possibility that the stars are early-AGB stars
polluted during the core He flash does not seem compatible with the theory.
