In this talk results of the study of the $D^0 to pi^- e^+ u$ decay channel, recorded by the BaBar detector at the c.m. energy close to 10.6 GeV, are reported. The branching fraction of this channel is measured relative to the $D^0 to K^- pi^+$ decay. The hadronic form factor, $f_{+,D}^{pi}(q^2)$, function of $q^2$, the four momentum transfer squared between the $D$ and the $pi$ mesons, is compared to various theoretical predictions, and the normalization $V_{cd} times f_{+,D}^{pi}(q^2=0)$ is extracted from a fit to data. Results are compared with Lattice QCD calculations. A new multipole model is applied which makes use of present information of resonant states contributing to the form factor. With the understanding of the $f_{+,D}^{pi}(q^2)$ form factor, and provided the relation between the $D^0 to pi^- e^+ u$ and $B^0 to pi^- e^+ u$ decay widths at the same pion energy, the CKM matrix element $V_{ub}$ is determined and compared to recent measurements. This method of extracting $V_{ub}$ will become competitive with new Lattice QCD calculations of the ratio of form factors.
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