Exclusive and kinematically complete high-statistics measurements of the basic double-pionic fusion reactions pn → dπ 0 π 0 , pn → dπ + π − and pp → dπ + π 0 have been carried out simultaneously over the energy region of the ABC effect using the WASA detector setup at COSY. Whereas the isoscalar reaction part given by the dπ 0 π 0 channel exhibits the ABC effect, i.e. a low-mass enhancement in the ππ-invariant mass distribution, as well as the associated resonance structure in the total cross section, the isovector part given by the dπ + π 0 channel shows a smooth behavior consistent with the conventional tchannel process. The dπ + π − data are very well reproduced by combining the data for isovector and isoscalar contributions, if the kinematical consequences of the isospin violation due to different masses for charged and neutral pions are taken into account
