From my book, Cinderella Tales From Around the World:
Histoire de la Belle Hélène de Constantinople is an epic verse romance—prose editions also exist—from northern France in the 15th century or even earlier. It is a romantic fiction based on a real person who has been sainted in Orthodox and Roman Catholic traditions as Saint Helena. Since it was too long to include in this collection, two summaries of the story are offered instead which highlight the Cinderella motifs. The tale is also closely related to ATU 706 The Maiden Without Hands, another tale type with a father incest motif.
As with so many historical figures who become legends, many of the stories surrounding Helena of Constantinople are fiction, especially this story. You can read about Helena and follow more links about her at Wikipedia, of course. Besides being long, the original is antiquated French, not easy reading for those fluent in French.
Also, Helena's story falls more into the ATU 510B Donkeyskin classification than ATU 510A. She escapes an unwanted marriage and proceeds from there. Well, I can share the text from Cox's preface to Cinderella that details the story:
Still more intricate are the events related in the French version (alluded to above), published in quarto, at Paris, without date, under the title: Histoire de la belle Heleine de Constantinople, mere de St. Martin de Tours en Touraine et de St. Brice son frere. Heleine is the daughter of Antoine, king of Constantinople, who married the sister of Pope Clement IX. Heleine's mother dies when she is fifteen years old, and, after remaining a widower for a time, the king asks his brother-in-law for permission to marry Heleine, for there is none as lovely as she. This the Pope, at first, refuses, though he had undertaken to grant any request Antonius might make, in return for his help in repulsing the Saracens; but soon after he consents, in accordance with divine command, which an angel brings. But this authority avails him nothing, for when Antoine reveals his intentions to his daughter, she throws herself at his feet weeping, and protesting that she would rather out off her hands and feet than suffer it. Then follow the flight and various adventures. Counselled by a nun, Heleine escapes in a Flemish ship to Sluis (Port de l'Ecluse), where she enters a convent. Antonius, in his rage, takes ship after her, and sails through every sea of Europe in vain quest. She lives for many a year in her retreat, till Cantebron, King of Sluis, who has become enamoured of her, directs his body guard of Saracens to storm the convent and carry her to his seraglio. Heleine flees in a Spanish ship sailing to Catalonia. But the ship is wrecked, and all save Heleine perish, she being cast ashore on the English coast. King Henry of England, taking his pleasure on the sea, is astounded at her beauty and the richness of her attire, and he rescues her. His offer of marriage she accepts, though she declines to reveal her descent, and will only say that she is "la plus noble Damoiselle de la Chrétienté". The marriage takes place against the wish of Henry's mother. Once more the Saracens threaten Rome, and Pope Clement seeks the aid of the King of Great Britain. He gives it in person, leaving the Duke of Gloucester as regent, and confiding Heleine to his care. Then follows the birth of the children, which the mother, who waylays the messenger at Dover, pretends are dogs, and the fraudulent letters. The Duke of Gloucester cannot make up his mind to burn Heleine, as the false letter directs, so, after cutting off one of her arms, for some unexplained purpose, he puts her to sea. A niece of the duke's, named Marie, offers herself to be burned with two straw dolls in the place of the queen and her sons. The hand of the queen, which had been cut off, is put in a box, and hung round the neck of one of the children. The boat lands them in Brittany. Whilst Heleine sleeps, a lion and a wolf from the forest make away with her children. She seeks them in vain, wandering at length to the neighbourhood of Nantes, where she takes refuge in a deserted hut, and lives on the alms of the passers-by. A hermit saves the children, and calls one Lion and the other Arm (Bras). Meanwhile, King Henry has slain the Saracens, freed Pope Clement, and returned to London, to learn the sorrowful fate of his wife and children. He is still bewailing his misfortunes, when Antonius, King of Constantinople, who has never ceased seeking his daughter, arrives on the scene. The two kings sympathise with each other, and discover that they grieve for the same person. The Duke of Gloucester reveals the truth, and, convinced of the guilt of the queen-mother, the king orders her banishment. London being hateful to him, Henry joins the Kings of Scotland and Constantinople in the war against the heathen of Europe. They first vanquish Clovis, King of Bordeaux, who allows himself to be baptised, and then joins in the crusade. The hermit, meanwhile, has brought up the children, and when they are sixteen years of age he sends them forth to discover, if possible, their parentage. They come to Tours, where the archbishop himself receives them, and changes the name of Lion into Martin, and of Arm into Brice. Heleine, too, comes to Tours, and receives rich alms from Martin, who does not know her. And the four kings come to Tours, where the two promising youths are presented to them. When the King of England opens Brice's box and sees the hand, he is convinced that he has found his two sons. Martin seeks the poor, one-handed woman whom he supposes to be his mother; but, on the arrival of the kings, she had fled in alarm over the Alps to Rome. Here she is supported by the Pope, her unknown uncle. Brice is taken to London, there to make manifest the innocence of his mother, and then goes with the four kings to Palestine to fight against the Saracens, whilst Martin remains at Tours with the archbishop. When the Saracens are subdued the conquerors journey to Rome, whereupon Heleine flees to Tours, revealing in a letter to the Pope that she is his niece. The King of England learns through this letter that his wife is still living, and is at length reunited to her. The archbishop of Tours permits Martin to place his mother's severed hand on the stump, and the two are united by a miracle. Antonius, with Brice and his wife Ludiene, goes back to Constantinople, Henry and Heleine live with Clement in Rome, and Martin remains in Tours, where he becomes archbishop.
Cox's preface discusses several contenders for medieval Cinderellas actually. I won't be blogging about them individually and overall did not discuss them in my book. The connections can be tenuous and the texts are long. Helena is one of the stronger candidates so I did highlight her. But in my research, Cox still provides one of the best overviews of medieval materials in connection to a Cinderella discussion.