Restricted login from WordPress main site to Django web app

I am looking to integrate a wordpress website at mydomain.com with a django web app set up at analytics.mydomain.com. The wordpress website is the main website, where payments are accepted, a user area are that contains the link to the Django analytics tools. The Django app is hosted on another site (e.g Heroku) but is set up as a subdomain.

I would like the database and user authentication and information on WordPress to act as a master to the Django slave rather than vice versa.

How can I integrate authenticated users on the wordpress site, who have paid for analytics functionality, to have access to the django app without requiring login in again, or how can I share the user logins automatically - like an single sign on or something similar?

What is the best way to go about this?