STOCKHOLM, Dec 20 (Reuters) - The European Union added three adult content companies - Pornhub, Stripchat and XVideos - to its list of firms subject to stringent regulations under new online content rules, the bloc's industry chief Thierry Breton said on Wednesday.

The new EU rules known as the Digital Services Act (DSA) require companies to conduct risk management, undergo external and independent auditing, and share data with authorities and researchers. (Reporting by Supantha Mukherjee in Stockholm; Editing by Andrew Heavens)