A real and tangible prize (taking a note from the Publius Enigma of Pink Floyd) for anyone who can unravel this mystery: a picture titled "AiAiAi".
This is as high-quality as I've got, and appearing here are unmistakeably Kago Ai, and Takahashi Ai, but who's that in the middle? This Aiko expert says that it is, in fact, Ms. Kayo. But this has not been corroborated. If it's anybody else, my guess is Tanaka Reina. The only problem is that Ms. Tanaka Reina appears to be at the right edge of the photo.
So the mystery is this: if this is indeed a photo of Kago Ai, Kayo Aiko, and Takahashi Ai, then why would Reina (who was chosen at an audition later than Aiko's) be there? In other words, what event is this from? H!P costume experts, I'm counting on you.
And if this is, in fact, from a later event, it's nice to see that Ms. Kayo gets along with the other girls who were lucky enough to be chosen, even a generation later.