Cello Strings

I'm trying to find out what kind of strings Yo Yo uses on his wonderful cello.
Mine is very old too, and I'm trying find out which one's he uses. The sound is amazing.
It's very expensive to order a new set of strings and then find out they sound lousy on your cello. You can't return them so I'm asking friends and other cellists which strings they use.
Thanks....
PSchiff