The rotation of comet C/1995 O1 Hale-Bopp from inner coma photometry

We present a further investigation of the periodogram resulting from the photometric data by Rodriguez et al. (1997) for comet C/1995 O1 Hale-Bopp and interpret that the main period in the data is 11.23 +/- 0.01 h, but not 7.19 days. The latter is now attributed to an alias of the 11.23-h period. Assuming that the periodicity observed in the photometry is the solar day, the 11.23-h period is consistent with estimates of the sidereal rotation period by Farnham et al. (1998), and Jorda et al. (1997-1999) provided that the obliquity of the comet's equatorial plane to its orbital plane is larger than 75 degrees and 80 degrees, respectively. This result is in agreement with estimates of the obliquity by Sekanina (1997-1999) and Jorda et al. (1998). A weaker periodic signal in the light curve could be 5.48 +/- 0.01 h, but we suggest that this is an alias of a 3.25 +/- 0.01 h period of unknown origin.