Referencing a column from another table in a different database with a foreign key constraint.

I am trying to write a foreign key table constraint which references
another column from a table in another database from within
PostgreSQL.

Within the database 'rsg_auth', there is a table called 'user_table'
which has a column (amongst others) called 'username'.

In another database called 'browser perm' I am trying to create the
following:

CREATE TABLE user_agreement ( ua_name    VARCHAR(16)    NOT NULL, ua_desc    TEXT        NOT NULL, sat_name
VARCHAR(20)       DEFAULT '', contract    VARCHAR(30)        DEFAULT '', filetype    VARCHAR(30)        NOT NULL,
areacode   VARCHAR(30)        NOT NULL, nrt    CHAR NOT NULL DEFAULT 'N' CONSTRAINT chkcons_nrt_y_n CHECK
 
(nrt IN ('Y', 'N')), PRIMARY KEY       (ua_name), CONSTRAINT fk_uaname_username        FOREIGN KEY (ua_name)       
REFERENCES rsg_auth.user_table(username)        ON UPDATE CASCADE      ON DELETE CASCADE
)

When I submitt it, I get the following error:   ERROR:  parser: parse error at or near "."

Obviously this has something to do with my reference to the other
table.

Can anyone put my syntax straight please?

Thanks in advance.

-- 

Mr. Daniel J. Smale
Remote Sensing Group
Plymouth Marine Laboratory