What is the highest year for the US Open?
CREATE TABLE "women_s_doubles_13_finals_13_titles" (
  "outcome" text,
  "year" real,
  "championship" text,
  "surface" text,
  "partner" text,
  "opponents" text,
  "score" text
);
SELECT MAX("year") FROM "women_s_doubles_13_finals_13_titles" WHERE "championship"='us open';