Bottom: anomalous Cepheids (squares) and Population II Cepheids (stars) in the M_V-period (P) plane. First-overtone AC have been highlighted with filled colour (green). It is not possible to make a clear classification between these two class of variables. We have plotted the cold (red) and the warm (blue) edges of the pulsation instability strip as derived by theoretical models (Fiorentino et al. 2006). Top: wesenheit plane, where the separation between the anomalous Cepheids and Population II Cepheids is clearer. The green solid line represents the relation for Population II Cepheids, and the black solid and dashed line represent the mass dependent Wesenheit relations for anomalous Cepheids, with masses 1.2 M_⊙ and 1.9 M_⊙ respectively. The distance modulus assumed is 24.33, as we found from RR Lyr.