If someone is born to a non-Jewish mother, and later converts, do we daven for them using ben/bat Sarah Imeinu or do we use their father's or father's mother's name?

Both the child and parent convert after birth?

My ex-wife converted (before we were engaged). She took the Hebrew name of Rifka & was called Rifka bas Sarah.

Generally, a convert's name is going to be X or Y ben/bath Abraham Abinou and Sarah Imenou. However, there are cases (e.g. having a Jewish father) where the convert would be X or Y ben/bath Father's Hebrew Name and Sarah Imenou.

do you have to say the "Avinu"/"Imenu" part? just "bat Sarah" is not enough?

I've heard the "avinu/imenu" part dropped from people I knew were gerim. Perhaps to preserve privacy.

If you are talking about a ger katan whose parents were also migayer I've heard of both. It is individual. For someone whose parents dud not convert they are always bas/ ben Avraham/ Sarah., You don't need the Avinu and Imenu.

The mother didn't convert, in this case. She died when her son was very young. He was raised Jewish and formally converted as an adult. Thanks for the answers.

I should have said if he was raise by Jewish parents. I've known adopted children who were migayer as babies who went by their adopted parents names. FWIW.

רפקה Isnt a hebrew name that i know of

It's a Yiddishized variant of Ribkah. I know plenty of Sepharade women whose only name is "Luna" or "Sol."

You mean Reebhqaa /reebhgaw right

Rivvvvvkah

no, Rivkah would be spelled רבכה but it isnt a kaf its quf and the vowel at the end is qawmas

Lol....only thing is, it's not yiddishized. I think it's more Americanized or just 'conversation'ized