Constraints on Form Factors For Exclusive Semileptonic Heavy to Light   Meson Decays
  We use rigorous QCD dispersion relations to derive model-independent bounds on the B -> pi l nu, D -> pi l nu and D -> K l nu form factors. These bounds are particularly restrictive when the value of the observable form factor at one or more kinematic points is assumed. With reasonable assumptions we find f_B < 195 MeV and that the shape of the form factor becomes severely constrained. These constraints are useful both for model discrimination and for model-insensitive extraction of CKM mixing parameters. 
C. Glenn Boyd, Benjamin Grinstein and Richard F. Lebed