During which event did Justin Bieber and Selena Gomez first officially out themselves as a couple?

Explanation

The rumours about a romantic relationship between Justin and Selena Gomez already started back in 2010. The two year older Selena was often to be seen at Justin's side, but both vehemently denied the rumours. Their first official outing as a couple was at the Vanity Fair Oscar Party on 27 February 2011. After their official debut as a couple, they were regularly seen with each other on holidays, etc. They broke up (again) in the end of 2012.