I had a distant cousin, Hilda Maud Armitage who married a John Zaharoff from Russia. They had four children, all born in England, of whom three died in infancy. Notes on Hilda that I was given by another distant cousin (now deceased) say that she died in Russia.
Their last child, Katherine, was born in 1922, and I have not found any trace of the family in British records after that date.
Assuming that Hilda, John and Katherine migrated to Russia, would there be any records accessible to make their biographies more complete?