I have two pieces of good evidence for this.
1. A member of the Sephardic-turned-Ashkenazic family Abugov (the Russified form of Abohab) studied in Dvinsk, a city which is now in Latvia under the name Daugavpils.
2. Autosomal DNA matching established Sephardic links to an Ashkenazic family from Rēzekne, another Latvian city, as I wrote in my article "Sephardic Jews in Lithuania and Latvia" in the August 2016 issue of ZichronNote, Journal of the San Francisco Bay Area Jewish Genealogical Society, after I received written permission from the living named participant:
"Judith Simon, a co-administrator of the two Iberian Ashkenaz projects at Family Tree DNA, grew up fascinated by the oral history related by her culturally Ashkenazic maternal grandfather, Shaya Brozgol (who changed his name to Sam Gold), that his ancestors on his father’s side included Sephardic Conversos who left Spain during the Inquisition. Brozgol was born in 1892 in Re_zekne, a city in eastern Latvia where his ancestors had also lived during the 1700s and 1800s, and married another Ashkenazic Jew from there. The family’s story of Sephardic heritage led some of Shaya’s cousins to move to Spain."
"Judith and several members of her family had their autosomal DNA tested, and two male paternal descendants of her Brozgol line had their Y chromosomal DNA tested. Family Tree DNA and GEDmatch provided matches that confirm the story. Judith, her brother, and her maternal aunt Pearl Freed share a triangulating identical-by-descent autosomal segment with seven Latin American Hispanics, and Pearl has several additional segments that match multiple Hispanics including Mexican-Americans with deep roots in northeastern Mexico and a Puerto Rican."
"The Brozgol Y-DNA lineage is also suggestive of Sephardic ancestry since not only does one of their closest matches (Belarusian Jewish) have an oral history that their paternal line came from the Ottoman Empire, but they also match Hispanics from Mexico and Texas whose most distantly known paternal-line ancestors centuries ago had Spanish first and last names. However, estimates vary widely on when the common Y-DNA ancestors of the Brozgol men and the Hispanics lived, making the autosomal results more definitive."
The total amount of Sephardic DNA in Litvaks is small - often no more than the average Mexican Catholic has - but finding them matching each other autosomally is powerful evidence supporting the genetic study Jan Meisels Allen posted to this group on 1/14/2019 in her message titled "(Latin America) Genetic Study of Latin Americans Reveals History of Converso Migration" that included some Mexican samples.
Kevin Alan Brook