PHOTOMETRIC-OBSERVATIONS OF THE NUCLEUS OF IC-418

We discuss the observational characteristics of a recently proposed cl ass of variable central stars of planetary nebulae. We summarize previ ous observations of the nucleus of IC 418, a prototype star of this cl ass, and we analyze new photometry of this object, obtained in 1989 an d 1991. We confirm that the characteristic timescale of the photometri c variations is a few hours. No single period can fit all of our obser vational runs, implying that the variability is either irregular or mu ltimodal. We discuss two possible explanations of the observed phenome na: (i) an unstable, radiatively driven wind and (ii) pulsational inst ability driven by the kappa-mechanism. We also consider the possibilit y that the observed variability is produced by a combination of both m echanisms and that they may influence each other.