  We estimate the sum of the $\Upsilon \bar BB $ couplings using QCD Spectral
Sum Rules (QSSR). Our result implies the phenomenological bound $\xi'(vv'=1)
\geq -1.04$ for the slope of the Isgur-Wise function. An analytic estimate of
the (physical) slope to two loops within QSSR leads to the accurate value
$\xi'(vv'=1) \simeq -(1.00 \pm 0.02)$ due to the (almost) complete
cancellations between the perturbative and non-perturbative corrections at the
stability points. Then, we deduce, from the present data, the improved estimate
$\vert V_{cb} \vert \simeq \ga 1.48 \mbox{ps}/\tau_B \dr ^{1/2}(37.3 \pm 1.2
\pm 1.4)\times 10^{-3}$ where the first error comes from the data analysis and
the second one from the different model parametrizations of the Isgur-Wise
function.
