Comment 8 for bug 968738

Here is what I was referring to:

http://en.wikipedia.org/wiki/Pluto#Charon

"The system is also unusual among planetary systems in that each is tidally locked to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured. Because of this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of gravity."