IU and Sung Si Kyung celebrated the success of their latest collaboration!

On December 9, the singers released their latest duet “First Winter.” (The two also previously collaborated on the 2010 track “It’s You.”)

That same night, IU shared her thoughts on Instagram, along with a photo of the song’s album cover art.

She wrote in the caption, “Following ‘It’s You,’ I sang with ‘Sung Balladeer’ [a popular nickname for Sung Si Kyung] again for the first time in almost 10 years. I participated without any hesitation, especially because it’s a song by Lee Kyu Ho, a senior artist of whom I’ve been a fan since I was young. The two of them did everything, and I was basically freeloading as I went along for the ride, but the one and only thing I did was suggest that we do a Christmas song. It’s a good thing I did that, right?”

IU then ended her post by writing to Sung Si Kyung, “Let’s take good care of our throats and meet again in 10 years.”

Later that night, Sung Si Kyung posted a screenshot of “First Winter” ranking No. 1 on Korean music sites Melon, Genie, Bugs, and Naver.

He added in the caption, “It’s embarrassing, but I guess I always wanted to post something like this. Since this was only possible because all of you listened to [the song], it’s not just a self-brag. Let’s be happy about it together, hehe.”

The singer went on, “I just finished practicing. It’s my sixth week of not drinking. I don’t plan on drinking up until the concert. I don’t know what a live broadcast is, but I’ll try doing it. Should I do it for about an hour on Wednesday around 8 p.m.? Old habits die hard. I’ll just do it in the format of a radio show with a live video stream in my room, and I’ll hold a Q&A and answer your questions. Please leave your stories or things you’re curious about in the comments.”

He added in the hashtags, “Thank you,” “It’s thanks to IU,” “I should buy her a rose gold necklace,” and “This is Instagram and Sung Si Kyung.”

IU responded to his post by writing the comments, “I’ll buy you a cashmere sweater.”

Meanwhile, TVXQ’s Changmin showed his support by leaving a comment with multiple thumbs-up emojis and writing, “Wow…so that kind of thing is possible to achieve. That’s amazing, hyung!”

Check out the music video for “First Winter” here!