What is the lowest number in series when U.S. viewers is 0.23 million?
CREATE TABLE "table1_29803475_2" (
  "no_in_series" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "u_s_viewers_million" text
);
SELECT MIN("no_in_series") FROM "table1_29803475_2" WHERE "u_s_viewers_million"='0.23';