  We study leptonic and semi-leptonic decays of D and B mesons. Use of the
Hopping Parameter Expansion (HPE) for two-point functions allows us
continuously to vary the pseudoscalar mass from below m_D up towards m_B. We
compute the pseudoscalar decay constants f_D and f_B, and observe consistency
with the value calculated in the static limit. {}From the measurement of
three-point functions we compute the matrix element relevant to the decay \bar
B -> D l \bar nu_l and extract the Isgur-Wise function xi(v.v'). The HPE
enables us freely to vary the initial state pseudoscalar mass at constant v.v',
and we investigate the 1/m_Q corrections to the heavy-quark limit.
