This paper provides a proof of the asymptotic sufficiency of the ranks in the two sample situation when the observations are censored at the $r$th order statistic. Several important conclusions may then be drawn concerning optimal tests.
"Asymptotic Sufficiency and Asymptotically Most Powerful Tests for the Two Sample Censored Situation." Ann. Statist. 4 (3) 589 - 596, May, 1976. https://doi.org/10.1214/aos/1176343464