Improved bilinears in lattice QCD with non-degenerate quarks
  We describe the extension of the improvement program for bilinear operators composed of Wilson fermions to non-degenerate dynamical quarks. We consider two, three and four flavors, and both flavor non-singlet and singlet operators. We find that there are many more improvement coefficients than with degenerate quarks, but that, for three or four flavors, nearly all can be determined by enforcing vector and axial Ward identities. The situation is worse for two flavors, where many more coefficients remain undetermined. 
Tanmoy Bhattacharya, Rajan Gupta, Weonjong Lee, Stephen R. Sharpe and   Jackson M. S. Wu