  In order to extract some information on the CKM matrix element |Vcb|, we have
to determine form factors in B decays. For this, we show that it is relevant to
consider the non-forward amplitude between the heavy-light B and D mesons
within the Heavy Quark Effective Theory. This method provides us bounds on the
shape of the elastic Isgur-Wise function. These bounds should be taken into
account in the parametrizations of the Isgur-Wise function used to extract
|Vcb|. Besides, we have also obtain new information on subleading functions at
the order O(1/mQ) where mQ is the heavy quark mass.
