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Hi, I have been trying to understand how much can we learn from reconstructing B->Dlnu decays on the recoil of fully reconstructed Bs. To this aim, I have been looking at the distribution of Mx in the D(*) mass range (1.8-2.1 GeV). In order to clean up the environment, I have requested: 1) either a K+ or a Ks 2) no neutrals 3) I have looked at B0 (D-lnu or D*-lnu, D*- -> D0pi-) and Bch (D0lnu) separately. The results are shown in http://babar.roma1.infn.it/~faccini/resoMx/resoVub.html I think we can conclude: a) that without kinematic fitting the resolutions on Mx in data and MC for tracks only are similar (see table at the bottom) b) that the measurement of the D0lnu and D+lnu events in our data show a bit of inefficiency that deserves more attention (although the stat is low...). Within the available statistics, resolutions and biases seem ok (maybe the D* is a bit strange) c) that the kinematic fit has a bad effect on these kinds of events. This is probably due to the fact that the pdf's used in it assume that there is a component with missing particles. In this case some events jump on the wrong part of the pdf and get nasty tails at high MX. This can be seen in babar.roma1.infn.it/~faccini/resoMx/fitNoFitD0lnu.eps where the noFit mass is plotted versus the fitted mass for D0lnu events in cocktail MC. The fact that the fit screws up "good events" is not necessarily a problem, but this means that the fitted mass cannot be used for Dlnu studies. d) after kinematic fitting the agreement between data and MC gets much worse, in particular for the cocktail. Kinematic fitting might be the origin of the fact that we need generic MC in order to get a reasonable agreement with the data. more to come (it looks like a promising sample) ciao ric