ABBA reunited temporarily for Claes af Geijerstam's 40th birthday party in 1986.

Claes confirmed this to be true and confessed that he even has it recorded on video! He said, "They apparently sang a specially-written lyric to the tune of the Chess song Der kleine Franz".

This is a privately-owned video (by Claes himself) so we must accept that we will never seen this!

Thanks to Magda at ABBA Planet (no longer online) and Markus.