Pictures taken on Sunday, 6 November show the Essex co-stars arriving on set together and being greeted by co-stars Chloe Sims and Liam 'Gatsby' Blackwell.
Frankie looks stunning in a floor length white gown and floral headpiece, while Mario is suited and booted in a dapper suit.
There's no word yet on why Frankie and Mario have returned to the show or whether they could be coming back next series. Frankie has confirmed via Twitter that she's appearing in the finale but hasn't said anything about staying on next series.
Frankie, who is Joey Essex's sister and Chloe Sims' cousin, left TOWIE in 2014 but has popped up a couple of times since.
A source told The Sun that Frankie, 29, has also filmed scenes showing her at Chloe's birthday brunch. "It seemed natural for Frankie to be there as they are so close – it was the perfect opportunity for her to dip her toe back into TOWIE life," said the insider.
"Frankie has been working extremely hard to get confident again with her body. Now she's lost weight and got healthier, it's given her a real boost and she feels it's the right time to revisit her old friends and be back on TV."
Frankie recently dropped from a size 16 to a size 8 after embracing a new healthy eating and exercise regime, and will spill her secrets in an upcoming weight loss DVD.
Mario, meanwhile, quit the series in August last year, but made a special appearance during the December Christmas special in a scene which saw him giving Billie Faiers' daughter Nelly her Xmas present.
TOWIE continues Wednesday at 10pm on ITVBe.
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