I searched the entire Internet to find this song from the movie La Piscine (1969). I don’t know whether it exists or maybe it’s just a short tune for the movie. In the party scene, two men are playing that song on the guitar and they are outside of the house by the pool. It's the scene right after Alain Delon opens the bottle at a party. Do you know what is the name of that song/tune?
It is approximately in the 41st or 42nd minute of the film and here is a link to a YouTube video with a timestamp:
Also, if you're not able to watch the video on YouTube, here's a clip: video file