They are DRMed - which, as far as I know, means they will only play in Overdrive (which allows an authorized transfer from Overdrive on the PC to Overdrive on the mobile device). I don't know of any other way to play them on my PC without Overdrive, so I suspect the same would be true for any mobile device...???
Something tells me I'd have to strip the DRM before I am able to play them by any other means, but that's just a guess.
I spoke to Darrell at Kobo about it and he's going to pass it on to the development team to get an answer for me, so we'll see what he says!!!