Looking for the name of this book series from the 90s or early aughts, and I can't remember the title, the author, or any of the characters names. What I can remember is that one of the girls had four arms, and on one of the covers she's dressed as a pirate, swinging on a rope. I remember that much because she was my favorite.
I don't remember if they were actually time-traveling or if it was a holodeck kind of situation, or why they were learning about Earth (unless, of course, they were trapped there via a faulty time travel device)
The other thing I remember is that there were Os in the title, because they were saturns/ringed planets on the covers.
ETA: something I remembered this morning: the books came with a sheet of temporary tattoos in them, which was a new big thing at the time.