I was listening to this lecture by Rav Herschel Schachter and near the end he parenthetically mentioned that there's an opinion of the Maharam MiRotenburg and Tosafos HaRosh in Yevamos that there's no kares if someone has adultery with an arusa (betrothed woman), only with a nesuah (fully married woman). He says besides being a surprising opinion, it's in conflict with the underlying premise of a gemarra somewhere.
This would make adultery with an arusa more strict than with a nesuah, as there is no atonement if done unwittingly (see Kesubos 37b).
Where is this Maharam MiRotenburg and Tosafos HaRosh in Yevamos? He didn't specify. As well, what gemarra is he referring to which has a premise against this novelty?