Wesenheit (V,   V − I) plane. Dots and triangles represent fundamental and first-overtone anomalous Cepheids as defined by OGLE-III (see Soszyński et al. 2008c). The star represents one P2C candidate from OGLE-III that we have included in our sample. The solid line represents the theoretical prediction for BL Her type stars from Di Criscienzo et al. (2007), while the dashed line is the observed Wesenheit relation for fundamental-mode classical Cepheids (Soszyński et al. 2008a).