Twinkle Twinkle Idol Star

Marimo Himenogi desperately wants to become an idol singer. When a local contest is announced, she begs her boyfriend, Ayumi to sing as her partner. Ayumi relents, and disguises himself as a girl so that he and Marimo can enter the singing competition. Surprisingly, they win and are signed to a recording contract as a new group called "Twinkle". Now they must keep their newfound fans from finding out that Ayumi is actually a boy.

(Source: Mangaupdates)