Divorced woman remarrying her ex-husband

Question

May a woman, who after receiving a get from her husband has relations with another man out of wedlock, remarry her husband?

Answer 1

Both the first relationship and the second one would have to be formal marriages (the kind that requires a Get to dissolve) for the prohibition of remarrying your divorcée to come into effect. (ShA EH 10:1)

איזו גרושה מותרת לחזור לבעלה ובו ז סעיפים:‏

המגרש את אשתו ואח"כ זינתה מותרת לחזור לבעלה:‏
הגה וכן אם זנתה עם הראשון ונתקדשה לשני וגירשה מותרת לחזור לראשון אפי' היתה מיוחדת לו תחילה (תשו' הרא"ש כלל נ"ג ומהר"ם פאדווה סימן י"ט)‏
אבל אם נתקדשה לאחר וגירשה או מת אסורה לחזור לראשון‏
ואפילו לא נתגרשה רק מכח קול בעלמא (הרא"ש כלל ה') (וע"ל סימן ו' וע"ל סימן מ"ו סעיף ה' וסעיף ו'):‏

The one who divorces his wife, and after that she committed adultery, she is permitted to return to her husband.
Ram''a: And this is also the case if she committed adultery with the first [husband], and she was betrothed to a second [husband], and he divorces her, she is permitted to return to the first [husband], even if she was designated to him first.
Shulchan Arukh: But if she was betrothed afterwards and he divorced her, or he died, it is forbidden for her to return to the first [husband].
Ram''a: And even if she was only divorced from the power of a mere rumor.