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.