The pulsation period Π as a function of evolutionary time τ (upper panel) and the rate of period change |$\dot{\Pi }$| as a function of period Π (lower panel) during the first crossing of the instability strip for the evolutionary sequence M_ZAMS = 5.9 M_⊙, α_ov = 0.1. In the upper panel solid lines show the second-order polynomial fits, open circles and filled circles show the periods of pulsationally stable and pulsationally unstable models, respectively. Open triangles show the periods of the secondary mode near the mode switching. In the lower panel, solid lines show time derivatives of analytical fits. Solid squares represent the time derivatives obtained by numerical differentiation.