Who is the woman a man is obliged to spend on, although she is not his wife, nor one of his mahrams, nor one of his heirs?

If a man has intercourse with a woman, believing her to be his wife, but she is married and her husband has not consummated the marriage, and she becomes pregnant by him, he is obligated to spend on her for the duration of the pregnancy until she gives birth. If she does not become pregnant, then he is obligated to spend on her for the duration of her 'istibra' (waiting period to ascertain non-pregnancy), which is three menstrual cycles for a free woman, according to some scholars.