  We follow the Galactic enrichment of three easily observed light n-capture
elements Sr,Y,and Zr.Input stellar yields have been first separated into their
respective main and weak s-process,and r-process components.The s-process
yields from AGB stars are computed,exploring a wide range of efficiencies of
the major neutron source,13C,and covering both disk and halo metallicities.AGBs
have been shown to reproduce the main s-component in the solar system.The
concurrent weak s-process,which accounts for the major fraction of the light
s-process isotopes in the solar system and occurs in massive stars by the
operation of the 22Ne n-source,is discussed in detail.Neither the main s-,nor
the weak s-components are shown to contribute significantly to the n-capture
element abundances observed in unevolved halo stars.We present a detailed
analysis of a large database of spectroscopic observations of Sr,Y,Zr, Ba,and
Eu for Galactic stars at various metallicities.Spectroscopic observations of
Sr,Y,and Zr to Ba and Eu abundance ratios versus metallicity provide useful
diagnostics of the types of n-capture processes forming Sr,Y and Zr.The
observed [Sr,Y,Zr/Ba,Eu] ratio is clearly not flat at low metallicities,as we
would expect if Ba,Eu and Sr,Y,Zr all had the same r-process origin.We discuss
our chemical evolution predictions, taking into account the interplay between
different processes to produce Sr-Y-Zr.We find hints for a primary process in
low-metallicity massive stars, different from the 'classical s-process' and
from the 'classical r-process',that we tentatively define LEPP (Lighter Element
Primary Process).This allows us to revise the estimates of the r-process
contributions to the solar Sr,Y and Zr abundances,as well as of the
contribution to the s-only isotopes 86Sr,87Sr,96Mo.
