PoS(LATTICE 2008)271 K -meson vector and tensor decay constants and B K -parameter from N f = 2 tmQCD ETM Collaboration P. Dimopoulos, R. Frezzotti, G.C. Rossi Dip. di Fisica, Università di Roma “Tor Vergata", Via della Ricerca Scientiﬁca 1, I-00133 Rome, Italy E-mail: {dimopoulos,frezzotti,rossig}@roma2.infn.it V. Gimenez Dep. de Fisica Teòrica and IFIC, Univ. de València-CSIC, Dr. Moliner 50, E-46100 València, Spain E-mail: vicente.gimenez@uv.es V. Lubicz Dip. di Fisica, Università di Roma Tre, Via della Vasca Navale 84, I-00146 Rome, Italy E-mail: lubicz@fis.uniroma3.it F. Mescia INFN-Laboratori Nazionali di Frascati, Via E. Fermi 40, I-00044 Frascati, Italy E-mail: mescia@lnf.infn.it S. Simula INFN-Roma Tre, c/o Università di Roma Tre, Via della Vasca Navale 84, I-00146 Rome, Italy E-mail: simula@roma3.infn.it A. Vladikas ∗ INFN-“Tor Vergata", c/o Dip. di Fisica, Università di Roma “Tor Vergata", Via della Ricerca Scientiﬁca 1, I-00133 Rome, Italy E-mail: vladikas@roma2.infn.it We present work in progress on the computation of the K-meson vector and tensor decay con- stants, as well as the B-parameter in Kaon oscillations. Our simulations are performed in a partially quenched setup, with two dynamical (sea) Wilson quark ﬂavours, having a maximally twisted mass term. Valence quarks are either of the standard or the Osterwalder-Seiler maxi- mally twisted variety. These two regularizations can be suitably combined in order to obtain a B K parameter which is both multiplicatively renormalizable and O(a) improved. The XXVI International Symposium on Lattice Field Theory July 14-19 2008 Williamsburg, Virginia, USA ∗ Speaker. c  Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/