[If] one remarries his divorced wife, or he marries his chalutzah, or the relative of his chalutzah, he must send her away, and the child [of that union] is a mamzer; [these are] the words of R' Akiva. The Sages, however, say: The child is not a mamzer. But they admit that if he marries the relative of his divorced wife, that the child is a mamzer.