UPSILON(B)OVER-BAR-B COUPLINGS, SLOPE OF THE ISGUR-WISE FUNCTION AND IMPROVED ESTIMATE OF V(CB)

We estimate the sum of the UPSILONBBBAR couplings using QCD spectral sum rules (QSSR). Our result implies the phenomenological bound xi'(upsilonupsilon' = 1) greater-than-or-equal-to -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'(upsilonupsilon' = 1) congruent-to -(1.00 +/- 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 \V(cb)\ congruent-to (1.48 ps/tauB)1/2(37.3 +/- 1.2 +/- 1.4) x 10(-3) where the first error comes from the data analysis and the second one from the different model parameterizations of the Isgur-Wise function.