  The formation mechanism of the barium stars is thought to be well understood.
Barium-rich material, lost in a stellar wind from a thermally-pulsing
asymptotic-giant branch star in a binary system, is accreted by its companion
main-sequence star. Now, many millions of years later, the primary is an unseen
white dwarf and the secondary has itself evolved into a giant which displays
absorption lines of barium in its spectrum and is what we call a barium star. A
similar wind-accretion mechanism is also thought to form the low-metallicity CH
and carbon-enhanced metal-poor stars. Qualitatively the picture seems clear but
quantitatively it is decidedly murky: several key outstanding problems remain
which challenge our basic understanding of binary-star physics. Barium stars
with orbital periods less than about 4,000 days should -- according to theory
-- be in circular orbits because of tidal dissipation, yet they are often
observed to be eccentric. Only one barium-star period longer than 10^4 days has
been published although such stars are predicted to exist in large numbers. In
this paper we attempt to shed light on these problems. First, we consider the
impact of kicking the white dwarf at its birth, a notion which is supported by
independent evidence from studies of globular clusters. Second, we increase the
amount of orbital angular momentum loss during wind mass transfer, which
shrinks barium-star binaries to the required period range. We conclude with a
discussion of possible physical mechanisms and implications of a kick, such as
the break up of wide barium-star binaries and the limits imposed on our models
by observations.
