According to online book store e-hon, a new OVA of Toaru Kagaku no Railgun
will be announced in the August issue of Dengeki Daioh. The official release date of the issue is June 26th.
The OVA is different from the previously announced extra anime "Episode 13': It's not easy to do a photo model under the burning sun." to be bundled with the official visual book.
Updated on June 23rd
According to Dengeki's news letter, the OVA will be released on October 29th.
Source: Scan of the notice