SUB-NATURAL LINEWIDTH COINCIDENT AUGER SPECTROSCOPY: APPLICATION TO Ar L1L2,3M1,2,3 COSTER-KRONIG TRANSITIONS F. Penent1, P.Lablanquie2, R.I. Hall1, M. Ahmad1, S. Diehl1, H. Kjeldsen3, J.H.D. Eland4, K. Ito5, Y. Hikosaka5, A. Muehleisen6, P. Pelicon6, Z. Smit6, M. Zitnik6 and F. Koike7 1DIAM, Universit=E9 P & M Curie, 75252 Paris Cx 05, France. 2LURE, Universit=E9 Paris-Sud, BP34, 91898 Orsay, F. 3Institute for Physics and Astronomy, Aarhus, Denmark. 4PTCL, Oxford University, Oxford OX1 3QZ, UK. 5 Photon Factory, Tsukuba 305, Japan. 6J. Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia. 7School of Medicine, Kitasato, Sagamihara 228, Japan A new experimental technique is presented based on threshold photoelectron/Auger electron coincidences that allows Auger lines, masked by energy broadening due to the initial hole lifetime, to be filtered and resolved. This constitutes an extension to above threshold of Resonant Auger spectroscopy. The efficiency of this technique is demonstrated by resolving the weak Ar L1L2,3M1,2,3 Coster-Kronig lines with a resolution down to 350 meV, far less than the Ar(2s-1) natural width (2.25 eV). The only limit to the resolution is that of the instruments.