Internal W-emmission and W-exchange Contributions to {\bf B}\to {{\bf   D}^{(*)} Decays
  We evaluate external $W$-emission, internal $W$-emission and $W$-exchange contributions to nonleptonic $B\to D^{(*)}$ decays based on the perturbative QCD formalism including Sudakov effects, whose ratio is found to be $1:+0.2:0.03i$ at the amplitude level. We observe that the internal $W$-emission contribution is additive to the external $W$-emission contribution, and that the $W$-exchange contribution is negligible and mainly imaginary, its real part being at least one order of magnitude smaller than the imaginary part. Our predictions are consistent with the CLEO data and with those obtained by the Bauer-Stech-Wirbel method. 
Chung-Yi Wu, Tsung-Wen Yeh and Hsiang-nan Li