Evidently it was a "third party tracking cookie" placed by one of the advertisers on this site. I blocked it using "tools". This is a statement I found about them:
"Third-party cookies are created by a Web site other than the one you are currently visiting; for example, by a third-party advertiser on that site. The purpose of such cookies is usually to track your surfing habits, which is why third-party cookies are considered an invasion of privacy and riskier than first-party cookies."
Blocking it has had no efect on my auto login.