Wandering near the red edge: photometric observations of three cool ZZ Ceti stars

We summarize our findings on three cool ZZ Ceti type pulsating white dwarfs. We determined eight independent modes in HS 0733+4119, of which seven are new findings. For GD 154, we detected two new eigenmodes, and the recurrence of the pulsational behaviour first observed in 1977. We discuss that GD 154 does not only vary its pulsations between a multiperiodic and a quasi-monoperiodic phase, but there are also differences between the relative amplitudes of the near-subharmonics observed in the latter phase. In the complex pulsator, Ross 808, we compared the pre- and post Whole Earth Telescope campaign measurements, and determined two new frequencies besides the ones observed during the campaign. Studying these stars can contribute to better understanding of pulsations close to the empirical ZZ Ceti red edge. All three targets are in that regime of the ZZ Ceti instability strip where short-term amplitude variations or even outbursts are likely to occur, which are not well-understood theoretically.