The mass-transfer scenario links chemical peculiarities with stellar
duplicity for an increasing number of stellar classes (classical and dwarf
barium stars, subgiant and giant CH stars, S stars without technetium, yellow
symbiotic stars, WIRRING stars, Abell-35-like nuclei of planetary nebulae...).
Despite these successes, the mass-transfer scenario still faces several
problems: What is the mass-transfer mode? Why orbital elements of dwarf barium
stars do not fully match those of the classical barium stars? What is the
origin of the few non-binary stars among dwarf barium stars? The paper reviews
these open questions.Comment: 14 pages, 4 figures, to appear in `Cosmic Abundances as Records of
Stellar Evolution and Nucleosynthesis', edited by F.N. Bash, T.G. Barnes, ASP
Conf. Ser., in pres