Hallo! Ouji-sama(1973)

18-years old japanese Miya Egawa helps a foreigner who is being chased by mysterious figures. As thanks he puts a ring on her finger, a huge ring embellished with huge diamonds and emeralds! And it won't come off!
As it turns out this foreigner was none other than crown prince Harold of the kingdom Cerulean! And now his handler claims the ring means that Miya has become his betrothed. Now Miya must head to Cerulean and clear up this misunderstanding. But there are dark forces after Harold's life and his right to the throne!! Will Miya safely make it back to Japan, or will she become a victim in the nefarious plot to take Harold's life?

Note: Volume 2 contains the one-shot Ohayou Mr. Baby (おはようMr. ベイビー)