The next three volumes of Attack on Titan manga are set to be offered in limited editions with OAD episodes of an adaptation of the Lost Girls stories about Mikasa Ackerman and Annie Leonhart, supervised by series creator Hajime Isayama. The latest issue of Bessatsu Shōnen Magazine,with the new chapter of the manga, has also offered a schedule of the planned episodes.

With Japanese release dates: 

Vol. 24: Annie’s story Part 1 (8 Dec 2017)

Vol. 25: Annie’s story Part 2 (9 Apr 2018)

Vol. 26: Mikasa’s story (9 Aug 2018)

Attack on Titan: Lost Girls was originally a pair of visual novels from Nitroplus and anime screenwriter Hiroshi Seko, released with volumes 3 and 6 of the original TV anime's Blu-ray release in 2013 (Mikasa's "Lost in the Cruel World"  and Annie's "Goodbye Wall Sina"). In 2014,  Seko expanded these into a prose novel (released in North America by Vertical). In 2015, it became a manga from Ryōsuke Fuji (released in North America by Kodansha Comics). Now, it's set to become an OAD, released with volume 24 (December 8th), 25 (April 9th) and 26 (August 9th) of the manga. 

via AoTWiki and Snk and Annie

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