Rooney Mara and Joaquin Phoenix have reportedly undergone a secret ''blessing ceremony''.

The 'Mary Magdalene' co-stars have been quietly dating for several months and with the 42-year-old actor preparing to go abroad for a six-month shoot with Jake Gyllenhaal on 'The Sisters Brothers', the couple decided to prove their commitment to one another with a low-key ritual.

A source said: ''They are faced with a long separation as he'll be shooting in either Australia or New Zealand. It's a looming threat.

''It's why they decided to have the ceremony. It was Joaquin's idea, just to show how committed he is to Rooney and to ease her nerves about him being away for so long.

''His pals and a small group of hers gathered in the garden at his house. It was all very simple.

''They exchanged promises to be a true couple and everyone threw flowers on them. Even though it's nothing legal, it was really sweet.

''You can tell she's really won his heart and no one is more surprised about it than him.''

While Rooney and Joaquin have gone to great lengths not to be photographed together until their recent appearance at the Cannes Film Festival, friends realised just how serious their relationship is when the 32-year-old actress moved to Los Angeles.

The insider told Grazia magazine: ''Rooney is smitten and she has broken her vow to never live in LA in order to be closer to Joaquin.

''All of her friends knew it was serious as soon as that happened because she's always hated LA.''

The couple - who met last year while shooting 'Mary Magdalene' - were first seen together leaving a spa in California in January, and have subsequently been spotted on lunch dates, but have never confirmed their relationship.

Though they attended Cannes together, insiders claim they refused to pose with the rest of their cast as they still don't want to go public with their love.