Matrix elements of electroweak penguin operators in the 1/N-c expansion

It is shown that the K --> pi pi matrix elements of the four-quark operator Q(7), generated by the electroweak penguin-like diagrams of the Standard M odel, can be calculated to first non-trivial order in the chiral expansion and in the 1/N-c expansion. Although the resulting B factors B-7((1/2)) and B-7((3/2)) are found to depend only logarithmically on the matching scale mu, their actual numerical values turn out to be rather sensitive to the pr ecise choice of mu in the GeV region. We compare our results to recent nume rical evaluations from lattice-QCD and to other model estimates. (C) 1999 E lsevier Science B.V. All rights reserved.