  Several stars at the low-metallicity extreme of the Galactic halo
([Fe/H]=-2.5) show strong enhancements of both s-process and r-process
elements. The presence of s-process elements in main-sequence stars is
explained via mass transfer from an AGB companion star in a binary system.
r-Process elements originate in type-II supernovae and also require mass
transfer. It is however unclear how pollution by both an AGB star and a
supernova could have occured. Here I show that the initial--final-mass relation
steepens at low metallicity, due to low mass-loss efficiency. This may cause
the degenerate cores of low-Z, high-mass AGB stars to reach the Chandresekhar
mass, leading to an Iben & Renzini-type-1.5 supernova. Such supernovae can
explain both the enhancement patterns and the metallicity dependence of the
double-enhanced halo stars. Reduced mass loss efficiency predicts more massive
remnants in metal-poor globular clusters. The evidence for a high M/L
population in the cores of globular clusters is briefly discussed.
