How would one authenticate to multiple cross-domain SPAs w/o relogin?

Hello all. I am creating a project with multiple microservices that provide or accept data for many different things.

We have a few SPAs that are meant to be linked, but separate. That is to say, you should not have to log in again when going between them (different domains, cross origin at the least). The caveat is however that we cannot share cookies (access token?).

How might this be done?