Evolution of the rate of change of the pulsation period of the fundamental radial mode, normalised to its instantaneous value, for models A (dashed-dotted curve), B (dashed curve), and C (solid curve) with an initial Z = 0.02 as a function of effective temperature. The shaded vertical region indicates the observed effective temperature range of the BLAPs. The dotted horizontal line indicates no change in period. The abscissa axis has been inverted to correspond to the H-R diagram; hence, time flows from right to left in the figure. Negative values on the ordinate axis indicate a shortening of the pulsation period, while positive values indicate a lengthening.