VARIABLE CENTRAL STARS OF YOUNG PLANETARY-NEBULAE .1. PHOTOMETRIC MULTISITE OBSERVATIONS OF IC-418

We report the results of a photometric multisite campaign devoted to H D 35914, the variable central star of the Planetary Nebula IC 418. Fro m the analysis of 120 hours of data acquired with a variety of techniq ues, we find that HD 35914 exhibits two distinct kinds of variability: irregular light modulation with a time scale of days, as well as cycl ic variations with a time scale of 6.5 hours. The short-term variation s are not strictly periodic, and cannot be reasonably explained by mul tiperiodicity; they appear to be semiregular. The star is generally re dder when it is brighter; this behavior appears to be connected with t he long-term variability. A re-analysis of most of the older data obta ined for HD 35914 by various researchers suggests that the basic behav ior of the star did not change during the last 15 years. We carefully discuss all the possible causes for the light variations of the star. Rotational modulation of surface features cannot explain the observati ons, and binarity is unlikely. Pulsations may be excited, but wind var iability (or a combination of both) can also not be ruled out.