Name the original air date for series number 15
CREATE TABLE "table1_26261072_1" (
  "series_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text
);
SELECT "original_air_date" FROM "table1_26261072_1" WHERE "series_num"=15;