[en] Correlations between positive pions are investigated in the target fragmentation region of 200 A GeV ¹⁶O+nucleus collisions. The pions are measured with the Plastic Ball detector in the WA80 experiment at the CERN SPS. The target mass dependence of the radii and the correlation strength extracted by interferometry is studied. A new approach to the fit of the correlation functions is introduced. The correlation strength and both invariant and transverse radii increase with decreasing target mass. The transverse radius for ¹⁶O+C reactions appears to be much larger than the geometrical radius of the nuclei involved. For the Au target only a small fraction of the measured pions contributes to the apparent correlation. Hints for a much larger second component in ¹⁶O+Au reactions are observed. Rescattering phenomena may provide a clue to understand these phenomena. (orig.)