From: Rabbi Moshe Sofer
To: Rabbi Aharon Segal
A charity collector confessed to repeated cohabitation with a non-Jewish woman and seeks a prescribed repentance regimen. The Chasam Sofer rules he must fast thirty-nine days per act of intercourse—corresponding to the thirty-nine lashes of malkus.
Source: Shu"t Chasam Sofer OC §175