Acording to this site Morning Musume'15 59th single is going to be released on August 19. The title is still unknown. More info to be added soon!
Thanks to reireimon on tumblr for the information.
I don't know if the site can be trusted, but I will edit the title and first post if the info is confirmed to be wrong.
UPDATE: Triple A-Side "Oh my wish! / Sukatto My Heart / Imasugu Tobikomu Yuuki". Release date 19th august confirmed via Hello! Station.
Will be released in three Regular and three Limited Editions.
Thread index:
page 109: Individual promotion videos + promotion on TV
page 111: Individual nationwide handshake event announcement video (details on p.112) + Imasugu Tobikomu Yuuki live performance
page 112: Sukatto My Heart PV making footage
page 124: Oricon Daily #1 with 114.181 copies sold on day 1. Congrats!
page 126: Sukatto My Heart live performance