The Residual Mass Term in the Heavy Quark Effective Theory
  We reformulate the heavy quark effective theory in the presence of a residual mass term, which has been taken to vanish in previous analyses. While such a convention is permitted, the inclusion of a residual mass allows us to resolve a potential ambiguity in the choice of the expansion parameter which defines the effective theory. We show to subleading order in the mass expansion that physical quantities computed in the effective theory do not depend on the expansion parameter. 
Adam F. Falk, Matthias Neubert and Michael Luke