Period trend ∣ β_P ∣ (left panel) and size of period fluctuations ΔP (right panel) as estimated from model (2) for the 53 Cepheids. Downward pointing triangles: control overtone Cepheids, upward pointing triangles: overtone twin-peak Cepheids, diamonds: fundamental-mode control Cepheids, squares: fundamental-mode twin-peak stars. In the left plot, negative trends are indicated in blue, positive ones in orange.