Even sets of four nodes on rational surfaces
  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). 
Alberto Calabri, Ciro Ciliberto, Margarida Mendes Lopes