When there is a separate mobile version of a site on a different URL to the main site, Google suggests to use their Annotations for desktop and mobile URLs.
On the desktop page, add:
<link rel="alternate" media="only screen and (max-width: 640px)" href="http://m.example.com/page-1" >
and on the mobile page, the required annotation should be:
<link rel="canonical" href="http://www.example.com/page-1" >
Can these be used cross domain?
Desktop site: www.example.com
Mobile site on a different domain: www.example.mobile
I know this is the possibly the worst way to set up a mobile version of a site, but say in a worst case scenario, could they work?
I'm thinking most likely not, however canonical tags can be used cross domain, so in theory it might work.