User still logged in after closing browser

in my app after a user logs in, if they close the browser and go back to the site they’re still logged in. I have duration set to 0, I thought that meant as soon as the session closes they are no longer logged in. I’m missing something obviously, can anyone steer me in the right direction here?

read about ‘CWebUser::allowAutoLogin’: http://www.yiiframework.com/doc/api/1.1/CWebUser#allowAutoLogin-detail

Cool thanks