  We have derived abundances of 33 elements and upper limits for 6 additional
elements for the metal-poor ([Fe/H] = -2.42) turn-off star HE 0338-3945 from
high-quality VLT-UVES spectra. The star is heavily enriched, by about a factor
of 100 relative to iron and the Sun, in the heavy s-elements (Ba, La, ..). It
is also heavily enriched in Eu, which is generally considered an r-element, and
in other similar elements. It is less enriched, by about a factor of 10, in the
lighter s-elements (Sr, Y and Zr). C is also strongly enhanced and, to a
somewhat lesser degree, N and O. These abundance estimates are subject to
severe uncertainties due to NLTE and thermal inhomogeneities which are not
taken into detailed consideration. However, an interesting result, which is
most probably robust in spite of these uncertainties, emerges: the abundances
derived for this star are very similar to those of other stars with an overall
enhancement of all elements beyond the iron peak.
  We have defined criteria for this class of stars, r+s stars, and discuss nine
different scenarios to explain their origin. None of these explanations is
found to be entirely convincing. The most plausible hypotheses involve a binary
system in which the primary component goes through its giant branch and
asymptotic giant branch phases and produces CNO and s-elements which are dumped
onto the observed star. Whether the r-element Eu is produced by supernovae
before the star was formed (perhaps triggering the formation of a low-mass
binary), by a companion as it explodes as a supernova (possibly triggered by
mass transfer), or whether it is possibly produced in a high-neutron-density
version of the s-process is still unclear. Several suggestions are made on how
to clarify this situation.

Astrophysics