John Travolta has taken to Instagram to wish his former Grease co-star Olivia Newton-John a "happy birthday", seven weeks after her death from breast cancer.MORE: John Travolta shares pride for daughter Ella as she makes her catwalk debutThe actor took to his Instagram Stories and alongside a still from their popular 1978 film, he simply wrote: "Happy Birthday my Olivia."WATCH: John Travolta shares heartwarming message in candid postOn what would have been Olivia's 74th birthday, her husband, John Easterling, 70, also shared a moving tribute.MORE: John Travolta pours his heart out about his family in emotional tributeREAD: John Travolta mourns loss of Grease co-star Olivia Newton-John following her deathTaking to his and her Instagram, John shared a beautiful photo of the pair looking happy whilst on holiday. "A birthday memory to @therealonj - swipe right to read: Happy Birthday Honey - I Love You! - Amazon John," he captioned the post.