STABILITY OF THE MULTIMODE PULSATION IN THE BETA-CEPHEI-TYPE VARIABLE-12 (DD) LACERTAE

Using all available photometric and spectrographic data of 12 (DD) Lac we studied secular stability of periods and amplitudes of all pulsati on 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 (P-1, P-4, and their combination, P-5) change in the same manner. Moreover, P-2 changes inversely to P-1. Such a phenomenon was previously found i n only one beta Cephei-type star, i.e. beta CMa. Observed period chang es 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 dia grams suggests that the mechanism(s) causing period changes probably d istinguishes modes with different l. The photometric amplitudes of the pulsation components do not vary, except for that of the f(2) compone nt, which shows an increase by about 30% during the last 20 years.