Halakhah sur Yevamot 1:8

And they, may their memory be blessed, also said (Yevamot 79b and see Mishneh Torah, Levirate Marriage and Release 6:8) about this matter that there are women that are exempt from levirate marriage and also from release. And these are them: the wife of a eunuch of the sun (caused by nature) or of an andrigonos; the wife of someone mentally incapacitated; the wife of a minor; a sterile woman; and one who is sexually forbidden to the levirate husband. As it is stated, "that his name may not be blotted out in Israel" - to exclude a eunuch of the sun and an andrigonos, the names of which are [already] blotted out; since they are not capable of having children from the beginning of their creation, they are like their own species. And it states, "The first son that she bears shall be" - to exclude a sterile woman, who is not capable of having children from the beginning of her creation. "The wife of the deceased shall not be married to a stranger, outside the family" - to exclude the wife of someone mentally incapacitated or a minor, that do not have marriage at all. "He shall take her as his wife" - to exclude one who is sexually forbidden to the levirate husband, that do not have taking (marriage) for him to them. And the sages counted (Mishnah Yevamot 1:1) fifteen women that do not have taking, that exempt their rival wives from release and from levirate marriage. And they said also (see Mishneh Torah, Levirate Marriage and Release 6:6-7) that there are some women that perform levirate marriage but do not get released, and there are some that get released but do not perform levirate marriage. And there are some that do not perform levirate marriage and do not get released. And there are from the brothers also those that are fit for levirate marriage and release, and some who are not fit for levirate marriage nor for release - and they have no connection [to the woman] at all - and there are some that are fit for release but they are not fit for levirate marriage, and there are some that are fit for levirate marriage but not for release. And the rest of the many details about these matters are all complete in the tractate connected to it and that is Tractate Yevamot.