Mont. Code § 40-1-404
A person who has cohabited with another to whom the person is not legally married in the good faith belief that the person was married to that person is a putative spouse until knowledge of the fact that the person is not legally married terminates that status and prevents acquisition of further rights. A putative spouse acquires the rights conferred upon a legal spouse, including the right to maintenance following termination of that status, whether or not the marriage is prohibited, as provided in 40-1-401, or declared invalid, as provided in 40-1-402. If there is a legal spouse or other putative spouses, rights acquired by a putative spouse do not supersede the rights of the legal spouse or those acquired by other putative spouses, but the court shall apportion property, maintenance, and support rights among the claimants as appropriate in the circumstances and in the interests of justice.
§ 40-1-404, MCA