Context :

Aims : We explore modifications to the current scenario for the slow neutron capture process in asymptotic giant branch ( AGB ) stars to account for the Pb deficiency observed in post-AGB stars of low metallicity ( [ Fe/H ] \simeq - 1.2 ) and low initial mass ( \simeq 1 - 1.5 { \mathrm { M } _ { \sun } } ) in the Large and Small Magellanic Clouds .

Methods : We calculated the stellar evolution and nucleosynthesis for a 1.3 { \mathrm { M } _ { \sun } } star with [ Fe/H ] = -1.3 and tested different amounts and distributions of protons leading to the production of the main neutron source within the ^ { 13 } C-pocket and proton ingestion scenarios .

Results : No s -process models can fully reproduce the abundance patterns observed in the post-AGB stars .
When the Pb production is lowered the abundances of the elements between Eu and Pb , such as Er , Yb , W , and Hf , are also lowered to below those observed .

Conclusions : Neutron-capture processes with neutron densities intermediate between the s and the rapid neutron-capture processes may provide a solution to this problem and be a common occurrence in low-mass , low-metallicity AGB stars .