This concerns all CERN retirees with property in France but who are not in the French national health insurance system (e.g. who do not have a carte "Vitale").
Certain CERN retirees who have revenue from property (e.g. rent) they own in France have been taxed on this revenue for the CSG (Contribution Sociale Générale). This tax is only due if the person concerned uses the French national health insurance system. Those who do not, irrespective of their nationality and where they live, are not subject to the CSG tax on revenue from property.
The CSG tax can be claimed back by using the template letter you can find on the page in French.
See French page.