The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds. A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena. One of these potentially non-linear pulsators. GD154, is on the red edge of the ZZ Ceti instability strip. It was first observed on May 1977 (Robinson et al. 1978). They obtained a power spectrum dominated by one mode (frequency F), its harmonics (nF) and 'half integer modes; ((2n +1)F/2). These observations are briefly discussed.