Left column: O−C diagram for RS Pup (top panel) and residuals of the model (bottom panel) for a linear period variation (red dashed line and points) and a fifth-degree polynomial function (blue solid line and points). The size of the black points in the upper panel is proportional to their weight in the fit (Table A.1). Right column: enlargement of the O−C diagram covering the last 400 pulsation cycles of RS Pup.