Periodic photmmetric variations in the near-nucleus zone of P/Giacobini-Zinner

We observed photometrically P/Giacobini-Zinner before its 1985 perihelion passage with the IHW standard filter set. We found a periodicity of 9.5 ± .2hr in the lightcurve of a color index of the nuclear region of the comet, which we interpret as being produced by albedo variations and also possibly by a nonspherical geometry of the spinning nucleus. A statistical analysis of our result indicates that the period we found is significant within at least a 97% confidence level. The same analysis, when applied to published data for P/d'Arrest, indicates that any periodicity there is significant to within no more than an 80% confidence level. The nucleus of P/Giacobini-Zinner is probably not smaller than 0.6 km.