Asteroid 271 Penthesilea is a main belt asteroid that was discovered by Viktor Knorre on October 13, 1887 in Berlin. The semi-major axis is 2.85 AU and the eccentricity is 0.102 with a fairly low inclination of 3.535 degrees.
I observed this asteroid on 5 nights during October and November 2012. My initial analysis of the data indicated a period of 20.800 hours, as shown in the first image below. However I was subsequently contacted by F. Pilcher who had also worked on this asteroid and had derived a period of 18.787 hours. A re-analysis of my data gave a result that was compatible with his as shown in the second lightcurve below. To confirm this, I obtained a further 4 nights of observations during December. These observations were only compatible with an 18 hour period. The 20 hour period was ruled out. This well illustrates one of the problems with asteroid photometry... when the period is a simple fraction of a day, it is very easy to get caught with an allias of the true period!