Problem with foreign keys I do 

ALTER TABLE bdt.auth 
    ADD CONSTRAINT `depositor no.` 
      FOREIGN KEY ( `depositor no.` ) 
         REFERENCES bdt.depositors( `depositor no.` ) 
            ON DELETE CASCADE 
            ON UPDATE CASCADE


I get


  Can't create table 'bdt.#sql-881_140' (errno: 121)


Fields are identical
There is no defined constraints:

SELECT
   constraint_name,
   table_name
FROM
   information_schema.table_constraints
WHERE
   constraint_type = 'FOREIGN KEY'
AND table_schema = DATABASE()
ORDER BY
   constraint_name;


Result


  Empty set (0.01 sec)
The error is that you're trying to use a constraint that is already identified.

SELECT
   constraint_name,
   table_name
FROM
   information_schema.table_constraints
WHERE
   constraint_type = 'FOREIGN KEY'
AND table_schema = DATABASE()
ORDER BY
   constraint_name;


Should give you the constraints you've already established. Also, you can use SHOW ENGINE INNODB STATUS or SHOW INNODB STATUS after you get the error to get the logs and more information.