I am being Misader Kiddushin for a couple in which the Chasan and Kallah are both Geirim. My question is regarding what to write in place of their father’s name?
You are to write Ben Avraham Avinu by the Chasan, and Bas Avraham Avinu by the Kallah. If, however, one writes only Ben/Bas Avraham without adding the word Avinu or Hager or Hagiyores, then the Kesuba is invalid and a new Kesuba must be written, just as we rule regarding a Ger by a Get.
Explanation: As by any legal document, properly writing the name identification of the person for whom the document is serving is of most importance, and if his name was not properly written, then it is invalid, as the document cannot prove anything regarding him, being that the name on it is not his. This applies likewise towards a Get and Kesuba, that it is critical that the names are written properly in order to identify the parties they are discussing. Now, from the letter of the law [according to most Poskim] it suffices to simply write the name of the individual without writing the name of his parent, and hence if a Get or Kesuba or any other legal document was written without mentioning the father’s name of the individual, it is valid. Nonetheless, the custom is to also write the father’s name in the document. Now, by a Ger the custom is to write Ben or Bas Avraham Avinu, as all the nations of the world can trace their lineage to Avraham who was called by God this name being that he would be the father of all nations. The Poskim, however, emphasize that one must add the word Avinu, or Hager, for this to be a valid identification, as if he simply writes Ben Avraham, then people will think that the person written on the document has a biological father whose name is Avraham and that he is not a convert, and hence such a document would be invalid.
Sources: See Otzer Haposkim 66:125; Kesuba Kehilchasa [Shtern] 6:13 and p. 255; Seder Kiddushin [Farkash] 3:12; Koveitz Hearos Ubiurim 864; Nitei Gavriel Nissuin 28:17 See regarding that the document is valid even if the father’s name is not mentioned: Michaber E.H. 129:9; Tur 129; Teshuvas Harosh 45:27; Levush 129:8 brings two opinions; Pischeiy Teshuvah 129:19 in name of Noda Beyehuda Tinyana E.H. 113 See regarding that custom is to also write the father’s name: Rama E.H. 129:9; Mahariy Viyal 23; See regarding a Ger that only their name is written and not anything further: Michaber E.H. 129:9; Rashba 2:17; Darkei Moshe 129:10 See regarding writing Ben Avraham Avinu by a Ger: Michaber E.H. 129:20; Beis Yosef 129; Teshuvos Harosh Kelal 9; Levush 66:2; 129:20; Nachalas Tzevi E.H. 129:13; See regarding writing Bas Avraham Avinu by a Giyores: Beir Heiytiv 129:35; Seder Haget Rashdam E.H. 108; Seder Kiddushin ibid; Nitei Gavriel ibid; Kesuba Kehilchasa ibid See regarding that if one wrote only Ben Avraham and did not write Avinu or Hager, that it is invalid: Beis Shmuel 129:39; Levush 129:20; Teshuvah Ashkenazis; Nachlas Tzevi 129:27; Beir Hagoleh 129 Nun; Beir Heiytiv 129:35; Kesuba Kehilchasa Teshuvah 29 p. 255