Media playback is unsupported on your device Media caption Oh Wonder perform their single Ultralife for the Radio 1 Piano Sessions.

Two and a half years ago, at the start of their career, Oh Wonder singer Josephine Vander Gucht fell over a gate, smashing her front teeth and breaking her nose.

It forced the band to remain anonymous for almost a year: No photographs, no interviews, no videos.

But there was an upside. Last year, as she caught a train home from Heathrow, she sat next to a passenger "covered in blood [with] no teeth, looking sorry for himself".

"I tentatively went up to him and his girlfriend and said, 'I just wanted to let you know you'll be fine'," recalls the 27-year-old.

"'Go to the dentist tomorrow, don't panic, you'll be great'. And he was like, 'Oh, thank you so much!'.

"And then this guy opposite us piped up, 'I broke my nose, too!'. And suddenly this whole little carriage was talking about their injuries, which was remarkable.

"When I got off the Tube, I was so excited. Chatting to strangers gives you such a buzz because there's that element of fear before you talk to someone.

"So I walked from Brockley station back to my house, singing into my phone. And I've got this really funny voice note, which is like, 'I'm getting high on humans!'"

Image copyright Josh Shinner Image caption Josephine Vander Gucht: "I would hate to be famous. I think it would just be very lonely."

Later, Josephine sent the melody to her musical partner, Anthony West, saying, "We have to write a song about this tomorrow".

"I just sent a text back saying, 'You're crazy,'" he laughs, but the demo was worked up into a full song, High on Humans, which features on the band's upcoming second album, Ultralife.

Like her Tube journey, the song tingles with nervous energy, capturing that extraordinary feeling of connecting with other people; a theme that runs through the record.

The duo were moved to write about "what it means to be a human in this day and age" after a head-spinning two years, in which their music became an online phenomenon, resulting in a record deal and a tour that ran to more than 200 shows around the world.

Best year ever

The success caught them completely off guard. Oh Wonder was conceived as a songwriting project, whereby the two musicians could subsidise their solo careers by giving songs to other artists.

In September 2014, they created a Soundcloud page and, prompted to describe themselves, wrote: "Writing duo, one song a month".

"We thought it was a good way to build up a portfolio over a year," explains Josephine. Then their first song, Body Gold, amassed 100,000 plays in just three days.

"We thought it was a fluke," says Anthony. "But then we uploaded something the next month, and the same thing happened. And it just kept snowballing.

"On the first of every month, we'd go to a little coffee shop and just release a song and thousands of people would play it. It was amazing. It was the best year ever."

Image copyright Josh Shinner Image caption The duo both released solo material before Oh Wonder took over their careers

Initially, they stayed incognito - partly because of their mantra "it's about the art, not the artist", but mainly because of Josephine's injuries.

"I had no teeth," she grimaces. "It was a really ill-timed accident."

"For a while, she had a little lisp," adds Anthony, who had to edit all of his partner's vocals to remove the erroneous "esses".

Recording on a budget of just £200, they hit upon a vocal style where both musicians sing in unison, with Josephine's voice in the centre, and Anthony singing twice, once in the left ear and once in the right.

The technique was actually a happy accident - they'd recorded two sets of vocals so the songs could be pitched to both male and female artists, but their managers advised them to blend the two takes, and it became Oh Wonder's unique sonic signature.

Fans, it seemed, couldn't get enough of it.

"Because we released a song a month over a long period, our fans had a year of their lives soundtracked by our music," says Josephine.

"Rather than just, 'Oh, I really like that song you did,' we get, 'You released that song when I was at university or I had moved to a different country'. We get the most heart-warming stories. Hundreds a week."

Anthony recalls a gig in Grand Rapids, Michigan, where a 50-year-old woman approached the band to express her gratitude for the band's music. Her son, who had recently died of cancer, had been a fan. In his final months, they would wait on tenterhooks for every new release.

"When he died, she was with him and they were listening to his favourite song," says Anthony. "And she said, 'I thank you so much because now, when I listen to that song, my son is there with me.'"

Stories like that helped alleviate the drudgery and loneliness of touring - "when you go from 3,000 people screaming your name to being back in the dressing room, on your own, eating a yoghurt."

Image copyright Getty Images Image caption The band have built a loyal fanbase in the US, and recently played a high profile slot at Coachella

Being whisked around the world has changed the two musicians in ways they never expected.

"I've found that I've become fiercely loyal now," says Josephine. "If someone has an engagement party and I've said I will be there, I will cancel everything, even if it really compromises things, in order to make it."

And that, in a nutshell, is what Ultralife - both the album and its title track - is about.

"You and I will both have days where we're really sad, like, 'it's Friday night and I'm in on my own and there's no food in the fridge and I'm miserable,'" says Josephine.

"And the next day you wake up and you're like, 'I feel fantastic, I feel so liberated and empowered and I have loads of mates and it's incredible.'

"Ultralife is the word we use to symbolise the someone or something in your life that pulls you out of those trickier moments and into the celebratory, ultra version of yourself."

So who is that person for Oh Wonder?

"We haven't been asked that yet!" laughs Josephine, before Anthony chips in with a vague answer about friends and family.

Many assume the duo are a real-life couple - but they have been doggedly reluctant to discuss a potential romance, even though they live together and have an uncanny knack for finishing each other's sentences.

"People think we're brother and sister, which suits us just fine," Josephine told one interviewer.

Image copyright Josh Shinner Image caption The band are on tour for the rest of the year, playing dates in Asia and America before returning home for six shows in October

Instead, they want the focus to be their music, which they wrote eyeball-to-eyeball over a piano in a rented AirB&B apartment in New York.

Compared to their first album, Ultralife is "pumped full of energy", a direct response to the audience's reactions on their last tour.

Sonically, it's a big leap forward, while retaining all of the breathless boy-girl beauty of their debut.

The band don't want the album to propel them to Beyonce levels of fame ("I think it would just be very lonely," says Josephine), but it could jeopardise their chances of striking up anonymous conversations on the Tube.

Which, as Anthony attests, could be a blessing.

"I once sat with a Danish friend on the Tube," he says, "And there was two girls opposite me talking in a foreign language.

"Later, my friend told me they were Danish, too, and they were tearing into my shoes, talking about how awful they were.

"I thought they were quite normal shoes, honestly."

Oh Wonder's new album, Ultralife, is released on 14 July by Island Records.

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