1994A&A...292..183P

Using all available photometric and spectrographic data of 12 (DD) Lac we studied secular stability of periods and amplitudes of all pulsation components of this star. By means of the LS periodogram analysis we confirmed the reality of all 6 previously known components. From the O-C diagrams we found all periods to be variable. Three of them (P₁, P₄, and their combination, P₅) change in the same manner. Moreover, P₂ changes inversely to P₁. Such a phenomenon was previously found in only one β Cephei-type star, i.e. β CMa. Observed period changes cannot be explained by a constant-rate evolutionary effect or by a light-time effect. However, a degree of regularity seen in the O-C diagrams suggests that the mechanism(s) causing period changes probably distinguishes modes with different l. The photometric amplitudes of the pulsation components do not vary, except for that of the f₂ component, which shows an increase by about 30% during the last 20 years.