We have modelled the multicycle evolution of rapidly-accreting CO white dwarfs ( RAWDs ) with stable H burning intermittent with strong He-shell flashes on their surfaces for 0.7 \leq M _ { \mathrm { RAWD } } / M _ { \odot } \leq 0.75 and [ Fe/H ] ranging from 0 to -2.6 .
We have also computed the i-process nucleosynthesis yields for these models .
The i process occurs when convection driven by the He-shell flash ingests protons from the accreted H-rich surface layer , which results in maximum neutron densities N _ { \mathrm { n,max } } \approx 10 ^ { 13 } – 10 ^ { 15 } \mathrm { cm } ^ { -3 } .
The H-ingestion rate and the convective boundary mixing ( CBM ) parameter f _ { \mathrm { top } } adopted in the one-dimensional nucleosynthesis and stellar evolution models are constrained through 3D hydrodynamic simulations .
The mass ingestion rate and , for the first time , the scaling laws for the CBM parameter f _ { \mathrm { top } } have been determined from 3D hydrodynamic simulations .
We confirm our previous result that the high-metallicity RAWDs have a low mass retention efficiency ( \eta \la 10 \% ) .
A new result is that RAWDs with [ Fe/H ] \la - 2 have \eta \ga 20 \% , therefore their masses may reach the Chandrasekhar limit and they may eventually explode as SNeIa .
This result and the good fits of the i-process yields from the metal-poor RAWDs to the observed chemical composition of the CEMP-r/s stars suggest that some of the present-day CEMP-r/s stars could be former distant members of triple systems , orbiting close binary systems with RAWDs that may have later exploded as SNeIa .