She plays the role of Claire Fraser in Starz’s Outlander series, alongside on-screen partner Sam Heughan.

And Caitriona Balfe proved her bond with her co-star is just as strong off-screen as she took toTwitterto share an amusing birthday tribute to the actor, who turned 37 on Monday.

In one hilarious tweet, the Irish actress, 38, penned: ‘Twitter just asked what’s happening…and I can safely say I have no idea!!!! Happy Birthday our kid @SamHeughan’.

The Super 8 star also shared a hilarious behind-the-scenes snap from their show, which showed the Doctors’ actor sticking his tongue out at an amused Caitriona.

The model also posted a meme of the Island at War actor, who joined thousands to run the 26-mile Stirling marathon on Sunday.

In the snap, the screen star sported a sweaty look, edited with the speech bubble: ‘I like getting my nails done!’

Alongside the meme, Caitriona penned: ‘Who da best..????? Always pamper yourself on your birthday. Especially if you’ve just crushed a marathon!!! @SamHeughan #ipromisei’llstopnow.’

Sam – who completed the race in three hours and 15 minutes – helped to raise £16,000 for Cahonas Scotland.

They pair been hard at work shooting season four of their critically-acclaimed show for the past few months after a gripping series three finale.

Viewers were left on the edge of their seats when Claire and Sam Heughan’s character James landed on the shores of America after battling an epic shipwreck in the season three finale of Outlander.

The season three finale opened with Claire submerged in the water with a rope around her in dramatic scenes.

‘Everything around me was a blinding white. And there was a soft rushing sound like the wings of angels. I felt peaceful and bodyless. Free of terror. Free of rage. Filled with a quiet happiness,’ she said in a voice over.

The nail-biting ending referenced the show’s beginning and saw Claire fighting for her life after nearly drowning in the Caribbean Sea.

But before it is too late James then jumped in and rescued her, with the pair then both washing up on a beach.

Season four is an adaptation of the fourth book in Diana Gabaldon’s series of novels – Drums Of Autumn.