Trilogy #3
If this turns out to be legit STAR WARS fans will have a Brit magazine named Hotdog to thank for it, but here goes: according to the December 2003 issue of that magazine, Chewbacca actor Peter Mayhew said that when he signed his contract to work in EPISODE III, there was a clause that he would be "required" to work in EPISODES VII, XIII and IX of the STAR WARS film series.
http://www.rickroot.com/blog/1/2005/05/Star-Wars-Episode-VI--Fall-of-the-Empire.cfm