Two Roman Catholic nuns who have publicly questioned their church's teaching on abortion said yesterday they will be expelled from their order, the Sisters of Notre Dame de Namur, unless they promise to discontinue such dissent.

The moderator general of the order gave the two women -- Sisters Barbara Ferraro and Patricia Hussey of Charleston, W.Va. -- until Feb. 15 to decide whether to accept the restrictions or face expulsion from the order.

"Your communications have given and continue to give scandal to Catholics because of the ambiguity of your pro-choice position," Sister Catherine Hughes, the moderator general, said in identical letters to the two women. "We consider such action . . . incompatible with membership in the congregation," she said in letters dated Jan. 15.

Ferarro and Hussey were among nearly 100 Catholics, 24 of them nuns, who in 1984 called for dialogue on the abortion question in a full-page, signed advertisement in The New York Times.

The Vatican Congregation for Religious and Secular Institutes ordered the superiors of the 24 nuns to demand that they recant or face expulsion from their religious orders. None of the nuns has publicly recanted, but all except Hussey and Ferraro have worked out with their orders compromises and "clarifying statements" that satisfied the Vatican.

In Hughes' letter, which the two nuns released along with their own statement yesterday, she said Hussey and Ferraro disregarded three conditions that the order had imposed on them pending resolution of the controversy:

That they "refrain from making any further public statements which do not fully convey your position so that the possibility of such statements being interpreted as pro-abortion will be minimized."

That they "refrain from using language which is abusive or disrespectful of persons."

That they consult with leaders of the order "before making further public statements on the subject of abortion" until the order "has developed a policy regarding dissent."

Since 1984 both Hussey and Ferraro, who run a program for the homeless, have participated in and addressed pro-choice demonstrations.

In their statement released yesterday, they said they were "deeply saddened" by the ultimatum, but a spokeswoman said neither "would talk to the press at this time."

"We believe the leadership's use of the vow of obedience to regulate a community member's public response on controversial issues will have a negative effect on the burden of conscience and moral agency of all members of the community," the two said. Ferraro has been a member of the order for 25 years and Hussey for 20.

The Oct. 7, 1984, advertisement came during a presidential election campaign in which New York Cardinal John O'Connor and other prelates criticized Democratic vice presidential candidate Geraldine Ferraro and New York Gov. Mario M. Cuomo (D), both Catholics, for defending the 1973 Supreme Court decision legalizing abortion.

The advertisement maintained that "a diversity of opinions regarding abortion exists among committed Catholics" and called for "candid and respectful discussion" of the question. It did not advocate abortion.

In their statement yesterday, the two nuns said, "According to the very limited protections we are afforded under canon law, we have until Feb. 15 to respond" to Hughes' letter. "If a second warning is issued at that time, we will have at least 15 days more at which time the leadership can, with the acquiescence of {the Vatican}, dismiss us."