La Chamade (aka Heartbeat)

Lucile, 25, is the beautiful mistress of Charles, a rich, good-hearted businessman. Being a kept woman suits her as she refuses to work. She is grateful to Charles for that but she does not feel true love for him. When she meets Antoine, a charming young man of her age, it is love at first sight. But living with Antoine is not as easy as it was with Charles…. Starring Catherine Deneuve and Michel Piccoli Directed by Alain Cavalier. Released 1968.
Heartbeat (1968) on IMDb