Sharing logins across Drupal sites - which module to use?

Hi

I am fairly new to the world of Drupal development.

I am working on a nature website of which there is another 'partner website'. My website and the partner website are both using Drupal 6. The sites are on different domains, different servers, different physical location and supported by different technical teams.

What both parties would like to achieve is the ability to share login ids between the two sites so that anybody who has created a profile and user account on one site can use this to login to the partner website.

I have done a bit of research and I am not quite sure what is the best route or module to use or if this is particularly easy to achieve. Here are some of the things that I have come across:

- Distributed authentication - this seems to be deprecated after Drupal 5 and seems to have security flaws
- Site Network module - this seems to be a copy of a module called 'drupal' and might do what I need... but may have security flaws.
- Web Services module - seems to offer some authentiction using OAuth... I am not sure if this will achieve what we need

We have OpenId logins running as well but this is something that is a separate approach to logins to the situation above.

Ideally we would like users who are logged in to one site to be able to click a link to a partner site and actually be logged in to the partner site as he or she arrives there from the original site. I am guessing that this might be quite difficult to achieve but if anybody has any ideas then that would be great.

The main issue is to enable user id/passwords from one site to be valid on the partner site.

Thanks in advance
Rich