Krau was a polite title used to address women, with "Trau" being the male equivalent. (TV: The Caves of Androzani) For example, Bernice Summerfield was addressed as "Krau Summerfield".
The title was used in business and other official circles in 2157 (PROSE: Lucifer Rising) and continued even in Galactic Federation culture in the 39th century. Women who were married and had children were not usually referred to as "Krau," a designation seemingly only for single women without children. (PROSE: Legacy)
In 2387, Veronique Hagen of the Guild of Adjudicators was addressed as "Krau Adjudicator." (PROSE: Infinite Requiem)