Thank you Neil !
Maybe I should have mentioned that I had already used the ObjRef utility and that in my case it shows no object reference. Nevertheless, reading the file that you linked too was of great help, because I just learnt this way about the _VFP.Objects collection, which was exactly what I needed ! When looping through it I can see than the objects that I thought released were not.
I suspect that the reason for which I see nothing in ObjRef is that it was written a long time ago, when _VFP.Objects was an array instead of a collection.
Now the icing on the cake would be a way to display to which session each of the objects is attached. For custom classes, I know I can write a method showing SET("Datasession"), but is there any way to achieve this even for VFP base classes ?

David