In each of the beautiful teasers, Lee Seo Yeon and Lee Na Kyung are focused on writing a letter before handing it to fans.
As previously reported, the 9-member girl group made their pre-debut with "Glass Shoes" after making it to the finals of Mnet's trainee survival show 'Idol School'. They'll now be making their official debut with a mini album 'To. Heart'.
'To. Heart' drops on January 24, so stay tuned!