row_to_json question

How can I use row_to_json for a subset of columns in a row? (without
creating a new view or using a CTE?)

What I want returned:
{"email_address":"joe(at)tanga(dot)com","username":"joevandyk"}
Note that there is no "id" column in the result.

create table users (id serial primary key, email_address varchar,
username varchar);
insert into users (email_address, username) values ('joe(at)tanga(dot)com',
'joevandyk');

select row_to_json(users) from users;
{"id":1,"email_address":"joe(at)tanga(dot)com","username":"joevandyk"}
Correct, except that the "id" column is in the result.

select row_to_json(row(users.email_address, users.username)) from users;
{"f1":"joe(at)tanga(dot)com","f2":"joevandyk"}
The column names are incorrect.