SharePoint 2013 - SharePoint Sites Authentication Prompt Between Sites in Browser Session

Hi,

SharePoint 2013 . 1 Application Server. Separate SQL Server. Patched up.

Published to the internet. Transitioning between sites prompts password challenge for that browser session (IE11).

Trusted Sites, Intranet Zone etc. doesn't fix it. Experience is seamless within the domain.

Fiddler just show's me the challenge. We need to add more sites in the future

Anyone seen this before

Could we overcome this with a separate WFE and some custom login form?