CREATE TABLE table_27978 ("No. in series" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" TEXT, "U.S. viewers  ( million ) " TEXT)

What episode number in the series was viewed by 13.66 million people in the U.S.?
SELECT MAX("No. in series") FROM table_27978 WHERE "U.S. viewers (million)" = '13.66'
SELECT MAX("No. in series") FROM "table_27978" WHERE "U.S. viewers (million)" = '13.66'