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We present an X-ray study of three mixed-morphology supernova remnants(SNRs), HB 21, CTB 1 and HB 3, using archival ASCA and ROSAT data. These dataare complemented by archival Chandra X-ray Observatory data for CTB 1 andXMM-Newton X-ray Observatory data for HB 3. The spectra from HB 21 and HB 3 arewell-described with a single-temperature thermal plasma in ionizationequilibrium, while a two-temperature thermal plasma is found in CTB 1. We foundenhanced abundances in all three SNRs. The elemental abundance of Mg is clearlyenhanced in CTB 1, while HB 21 has enhanced abundances of Si and S. Thesituation is not so clear in HB 3 -- the plasma in this SNR either hassignificantly enhanced abundances of O, Ne and Mg, or it has marginallyenhanced abundances of Mg and under-abundant Fe. We discuss the plausibility ofmixed-morphology SNR models for the three SNRs and the presence of enhancedabundances. We revise a list of MM SNRs and their properties, compare the threeSNRs studied here with other members of this class, and discuss the presence ofenhanced elemental abundances in MM SNRs. We also report the ASCA detection ofa compact source in the southern part of HB 3. The source spectrum isconsistent with a power law with a photon index of ~2.7, and an unabsorbedX-ray flux of ~10^{-12} erg/cm^2/s in the 0.5--10.0 keV band. The columndensity towards this source differs from that towards the SNR, and it istherefore unlikely they are related.Comment: 26 pages, 15 figures, revised version (minor changes), accepted for  publication in ApJ (10 Aug 2006

Enhanced Abundances in Three Large‐Diameter Mixed‐Morphology Supernova Remnants