  CP asymmetries in B_d \to D^+D^-, D^{*+}D^-, D^+D^{*-} and D^{*+}D^{*-}
decays are investigated with the help of the factorization approximation and
isospin relations. We find that the direct CP violation is governed only by the
short-distance penguin mechanism, while the indirect CP asymmetries in B_d \to
D^{\pm}D^{*\mp} transitions may be modified due to the final-state rescattering
effect. An updated numerical analysis shows that the direct CP asymmetry in
B^0_d vs \bar{B}^0_d \to D^+D^- decays can be as large as 3%. The CP-even and
CP-odd contributions to the indirect CP asymmetry in B^0_d vs \bar{B}^0_d \to
D^{*+}D^{*-} decays are found to have the rates 89% and 11%, respectively. Some
comments on the possibilities to determine the weak phase \beta and to test the
factorization hypothesis are also given.
