Age–metallicity relation for SMC WH star clusters. Stars denote the results from this work, and triangles indicate the results from the extended sample from the literature (see text and Tables 1 and A.2 for more details). The extended curves represent the results of the model in which the merger has mass ratios of 1:4 (black solid curve) and 1:2 (blue dashed curve). The green solid curve represents the merger model with a 1:4 mass ratio that took place 6 Gyr ago and caused an enhanced SFR, whereas the red solid curve is the scenario with an enhanced SFR 2 Gyr later.