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We present the results of a comparative period search on differenttime-scales and modelling of the ZZ Ceti (DAV) star GD 154. We determined sixfrequencies as normal modes and four rotational doublets around the ones havingthe largest amplitude. Two normal modes at 807.62 and 861.56 microHz have neverbeen reported before. A rigorous test revealed remarkable intrinsic amplitudevariability of frequencies at 839.14 and 861.56 microHz over a 50 d time-scale.In addition, the multimode pulsation changed to monoperiodic pulsation with an843.15 microHz dominant frequency at the end of the observing run. The 2.76microHz average rotational split detected led to a determination of a 2.1 drotational period for GD 154. We searched for model solutions with effectivetemperatures and log g close to the spectroscopically determined ones. Thebest-fitting models resulting from the grid search have M_H between 6.3 x 10^-5and 6.3 x 10^-7 M*, which means thicker hydrogen layer than the previousstudies suggested. Our investigations show that mode trapping does notnecessarily operate in all of the observed modes and the best candidate for atrapped mode is at 2484 microHz.

Multimode pulsation of the ZZ Ceti star GD 154