On October 9, Lee Joon and Thunder surprised fans by sharing photos of them together on Instagram. Lee Joon pointed out Thunder's perfect body proportions and captioned, "I guess this is what it means to have a small head."

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A post shared by ?? leejoon (@leechangsun27) on Oct 9, 2017 at 4:52am PDT

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Thunder also shared a photo taken with Lee Joon on his Instagram and excited fans with the caption, "Recording finished..!", hinting at a possible collaboration. 

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A post shared by ?? THUNDER (@thunderslighthouse) on Oct 9, 2017 at 4:59am PDT

Meanwhile, Lee Joon is scheduled to enlist in the military on October 24. Are the two stars preparing a surprise for fans before Lee Joon's enlistment? 

Stay tuned for updates!