  The recently measured semileptonic D_s decay rate can be used to constrain
weak annihilation (WA) effects in semileptonic D and B decays. We revisit the
theoretical predictions for inclusive semileptonic D_{(s)} decays using a
variety of quark mass schemes. The most reliable results are obtained if the
fits to B decay distributions are used to eliminate the charm quark mass
dependence, without using any specific charm mass scheme. Our fit to the
available data shows that WA is smaller than commonly assumed. There is no
indication that the WA octet contribution (which is better constrained than the
singlet contribution) dominates. The results constrain an important source of
uncertainty in the extraction of |Vub| from inclusive semileptonic B decays.
