We exploit the uniform age and initial chemical composition of the halo common proper motion pair, HD 134439 and HD 134440, to eliminate metallicity- and age-dependent effects in an investigation of Li processing in cool, metal-poor dwarfs. We have obtained a very high S/N (480) Keck observation of Li in HD 134440 and derive a 3 σ upper limit on its lithium abundance of A(Li) ≤ -0.59. This contrasts with an abundance over 1 dex higher, A(Li) = 0.45 ± 0.05, in HD 134439, which is hotter by only 200 K. This difference verifies that Li processing in cool, metal-poor dwarfs is very sensitively dependent on effective temperature. Additionally, the stark contrast between Li in HD 134440 and in the tidally locked binary star BD - 0°4234, reinforces the significance of the detection in the latter, which may lend support to the suggestion that rotationally induced mixing is a significant mechanism for Li depletion.
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