How to log into joomla through an external script?

Question

We have a standalone script on our site that sits adjacent to a Joomla 1.5 installation. We are using Joomla authentication to restrict access to our script. At this point we are redirecting any unauthorized users to the Joomla site to log in. We want to add a login capability within our script, though. Does anyone know how to log into joomla from an external script using a username/password? Thanks!