Tales of the Legion of Super-Heroes #350 Newsstand

The Emerald Empress has gathered her own Fatal Five and intends to use the team to accomplish the goal of conquering the universe and destroying the Legion of Super-Heroes. Element Lad sends a team of Legionnaires to follow Sensor Girl because he feels that he may have sent her to join the Fatal Five after he shared that the team was suffering dissension due to her not wanting to reveal her true identity. While they follow Sensor Girl's trail, thanks to Dawnstar's tracking ability, another team of Legionnaires, consisting of Colossal Boy, Dream Girl, Tellus and Polar Boy, head to Titan to search for Mentalla, a student of the Legion Academy who abruptly left. Their search takes them to the planet of Stratus, where they find four members of Fatal Five. The Legionnaire's spacecraft is destroyed immediately and soon Sensor Girl arrives to the scene of the fight. While the Legion is in doubt if she is there as a teammate or as a foe, she suddenly attacks the Fatal Five and their fifth member is revealed: Mentalla. Emerald Empress gets the upper hand in the battle and is able to reveal Sensor Girl's secret identity: Princess Projectra.