Sometimes, academic journal articles are made available online prior to being formally published in an academic journal. These are often referred to as preprints and may not yet have gone through a formal peer-review process.
To reference a prepublication journal article found online or in digital repositories you will need:
Author surname, Initials. (Year) Title of article. To be published in Title of Journal [Preprint]. DOI: doi number/Available from: URL [Accessed date]
Jeon, I., Lee, K. and Park, J.-H. (2015) Ramond-Ramond cohomology and O(D,D) T-duality. To be published in Journal of High Energy Physics [Preprint]. Available from: http://arxiv.org/abs/1206.3478 [Accessed 24 August 2015].
(Jeon et al., 2015)