  We introduce an effective lagrangian including negative and positive parity
heavy mesons containing a heavy quark, light pseudoscalars, and light vector
resonances, with their allowed interactions, using heavy quark spin-flavour
symmetry, chiral symmetry, and the hidden symmetry approach for light vector
resonances. On the basis of such a lagrangian, by considering the allowed weak
currents and by including the contributions from the nearest unitarity poles we
calculate the form factors for semileptonic decays of $B$ and $D$ mesons into
light pseudoscalars and light vector resonances. The available data, together
with some additional assumptions, allow for a set of predictions in the
different semileptonic channels, which can be compared with those following
{}from different approaches. A discussion of non-dominant terms in our
approach, which attempts at including a rather complete dynamics, will however
have to wait till more abundant data become available.
