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We describe smooth rational projective algebraic surfaces X, over an algebraically closed field of characteristic different from 2, having an even set of four disjoint (−2)-curves N 1 , . . . , N 4 , i.e. such that N 1 + · · · + N 4 is divisible by 2 in Pic(X).doi:10.4310/mrl.2004.v11.n6.a7 fatcat:ug726igjnve35pdnqwifskldhu