  The electronic structure of (Ce,Yb)CoIn5 has been studied by a combination of
photoemission, x-ray absorption and bulk property measurements. Previous
findings of a Ce valence near 3+ for all x and of an Yb valence near 2.3+ for
x>0.3 were confirmed. One new result of this study is that the Yb valence for
x<0.2 increases rapidly with decreasing x from 2.3+ toward 3+, which correlates
well with de Haas van Alphen results showing a change of Fermi surface around
x=0.2. Another new result is the direct observation by angle resolved
photoemission Fermi surface maps of about 50% cross sectional area reductions
of the \alpha- and \beta-sheets for x=1 compared to x=0, and a smaller,
essentially proportionate, size change of the \alpha-sheet for x=0.2. These
changes are found to be in good general agreement with expectations from simple
electron counting. The implications of these results for the unusual robustness
of superconductivity and Kondo coherence with increasing x in this alloy system
are discussed.
