Flavour Dependence of Form Factors in Heavy Meson Decays

Recently we have shown that due account of Wigner spin rotations is needed to match the ISGW model consistently onto HQET. We now discuss the flavour dependence of this procedure. We find that for finite mass ``heavy'' quark the universal function $\xi(y=1) \approx 1$ but the slope is proportional to the energy of the spectator quarks, and ratios of form factors are corrected in a way that QCD sum rules seem to require. Predictions for heavy quarks, such as in $B_c \rightarrow \psi(\eta_c)$ and to mixed systems, such as $D\rightarrow K$ and $B \rightarrow \pi$ show a systematic mass dependence that may be confronted with data.