The decays t+cV, where V = y , g, or Z , and t+cH are calculated, keeping all masses, in the standard model (SM) for all possible values of the top-quark mass that are consistent with present data. We find that the branching fractions for these processes are small, the largest being B ( t-+CHIand B ( t +cg ) -1 0 l O . Our calculations are then extended to include the possible contributions of charged Higgs bosons to the transitions t -+cV in the context of general two-Higgs-doublet models. These new contributions can enhance the SM branching fractions by as much as 3 4 orders of magnitude, e.g., B ( t -c g ) -10-~-10-~, for various values of the parameters. General expressions for flavor-changing neutral-current decays of heavy quarks including all masses and momenta, in the SM and in the two-Higgs-doublet models, are also given.

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Eilam, Hewett, and Soni Reply:We agree with the arguments of Scares [1], which may have practical consequences for extensions of the standard model. Indeed, in our Eq. (6) [2] for the CP violation asymmetry (i.e., a) for the /-• qq'q mode, Vw in the second line of the equation should be replaced by V'w, where T'w^Tw -ViW-* q'q) to take account of the rescattering effects. However, as discussed in our Letter, the largest CP violation asymmetry in the standard model is obtained for the process /-• dcd for which numerical results were presented. For that process, rescattering eflfects are very small