Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_19236587_4 ( original_air_date VARCHAR, series_no VARCHAR )

### Question ###
How many different original air dates does the episode with series number 13 have?

### Accurate SQL ###

SELECT COUNT(original_air_date) FROM table_19236587_4 WHERE series_no = 13