6 Summary and conclusions

We carried out a time-series photometric study of the variable central star of the young Planetary Nebula M 2-54. The behaviour of this object is strikingly similar to that of the best studied representative of this class of variable star, HD 35914, the central star of IC 418. The similarity between M 2-54 and HD 35914 strongly suggests that the physical reason causing the variations is the same.

Our light-curve analysis showed that slow, apparent nonperiodic, light variations with a time scale of days and short-term variability with a time scale of several hours is present in M 2-54. More specifically, the faster variations are (quasi)periodic with a time scale of either 8.9 or 14.3 hours. We cannot definitely distinguish between these two possibilities because of aliasing ambiguities.

While the long-term variations of M 2-54 may be explained by both a spot or a wind-variation hypothesis, the short-term variations are most likely due to stellar pulsation, suggesting that we are in the presence of a new class of pulsating star.

However, further work is needed to confirm or reject this claim. Observationally, the temporal behaviour of a number of central stars with well-known basic parameters, e.g. from model atmosphere analysis (e.g. Méndez et al. 1992) should be studied in more detail. Multisite observing campaigns (which do not require more than two or three sites well separated in longitude) during a time span of several weeks need to be undertaken. Besides time-series photometry, spectroscopic investigations would be very useful.

On the theoretical side, pulsational stability analysis, similar to that of Gautschy (1993) is required for a set of models of different mass and chemical composition. Close collaborations between theorists and observers are important for these stars being investigated in a satisfying manner.

Acknowledgements

This work was partially supported by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung under grant S-7304. The author thanks S. Zola, J. Krzesinski and W. Ogloza for attempting to obtain data of M 2-54 in the same period of time as ours (and deeply regrets their bad luck with weather). The referee is thanked for constructive criticism which helped to improve the quality of this paper.