SIMBAD references

We discuss the observational characteristics of a recently proposed class of variable central stars of planetary nebulae. We summarize previous observations of the nucleus of IC 418, a prototype star of this class, and we analyze new photometry of this object, obtained in 1989 and 1991. We confirm that the characteristic timescale of the photometric variations is a few hours. No single period can fit all of our observational runs, implying that the variability is either irregular or multimodal. We discuss two possible explanations of the observed phenomena: (i) an unstable, radiatively driven wind and (ii) pulsational instability driven by the κ-mechanism. We also consider the possibility that the observed variability is produced by a combination of both mechanisms and that they may influence each other.