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 .