Sheikh Al ‘Uthaymīn said, “if two disbelievers get married, then the wife becomes Muslim, if it is before the marriage is consummated, the marriage is invalidated. If she becomes Muslim after the marriage is consummated, the marriage is not invalidated, but they wait: If the man becomes Muslim before the ‘iddah (waiting period) is over, she is his wife. If the ‘iddah comes to an end and he has not yet become Muslim, she is not his wife because it is then clear that the marriage has been invalidated since she became Muslim. [Sheikh Al ‘Uthaymīn, Ḥukm Tārik Al-Ṣalāt (Rulings regarding those who abandon prayer), p.28]