I've personally encountered many ringed Ys, Ts, and Ls, a few ringed Ms, WDs, two NS, and one ringed K. I've seen someone else's screenshots of a ringed G.
A few things:
1. As far as I've seen stars with visible rings are always child objects, so your best bet for hunting them is in Herbig Ae/Be systems and black holes.
2. An interesting note: asteroid belts are considered rings (you can see this when you look at the scan entry in the journal for a body with an asteroid belt.)
3. Sometimes the star may have an invisible ring. It is still there! Just very sparse so you can't really see it very easily. If you use the semi-major axis numbers from the journal you can fly out to the approximate location of the ring then inch verrrrrry slowly (<1Mm/s) toward to to drop in. The ring will usually be rotating on the same plane as other objects orbiting the star the ring is around.