We have made a parametric study of s -process nucleosynthesis in the metal poor ( [ Fe/H ] = -2.7 ) stars LP625-44 and LP706-7 .
We find that a high neutron exposure and a small overlap factor are necessary to fit the abundance pattern observed in these two metal-deficient stars , particularly the abundance ratios , Pb/Ba \approx 1 and Ba/Sr \approx 10 .
We have also constructed stellar models to better understand how such s -process conditions could arise .
We have calculated a 2 M _ { \odot } model star with metallicity [ Fe/H ] = -2.7 from the ZAMS up to AGB phase .
We find that for such low-metallicity stars the He convective shell reaches the bottom of the overlying H-rich envelope at the second thermal pulse .
Protons are then carried into the hotter He burning layers and ^ { 13 } C is formed as protons mix into the He shell .
Subsequently , material in the H-flash driven convective zone experiences a high neutron exposure due to the ^ { 13 } C ( \alpha , n ) reaction .
This results in a new neutron-capture s -process paradigm in which the abundances are characterized by only one neutron exposure .
We suggest that this new s -process site may be a significant contributor to the s -process abundances in low-metallicity ( [ Fe/H ] \leq - 2.5 ) stars .

KEYWORDS : s-process , AGB stars , lead , nuclear reactions