CREATE TABLE table_31355 ("No. in series" FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "U.S. viewers  ( million ) " TEXT)

What is the lowest number in series when U.S. viewers is 0.23 million?
SELECT MIN("No. in series") FROM table_31355 WHERE "U.S. viewers (million)" = '0.23'
SELECT MIN("No. in series") FROM "table_31355" WHERE "U.S. viewers (million)" = '0.23'