Effect of m_c on b quark chromomagnetic interaction and on-shell   two-loop integrals with two masses
  The effect of non-zero c quark mass on b quark HQET Lagrangian, up to 1/m_b level, is calculated at two loops. The results are expressed in terms of dilogarithmic functions of m_c/m_b. This calculation involves on-shell two-loop propagator-type diagrams with two different masses, m_b and m_c. A general algorithm for reducing such Feynman integrals to the basis of two nontrivial and two trivial integrals is constructed. 
A.I. Davydychev and A.G. Grozin