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