What is the memory overhead of a .NET Object

What is the memory overhead of an Object in .NET? I'm talking about an arbitrary bare-bones object.... the overhead of the internal .NET workings or references:

var obj = new System.Object();

How much space does obj occupy in the heap?

I talk about this in a blog post "Of memory and strings". It's implementation-specific, but for the Microsoft .NET CLR v4, the x86 CLR has a per-object overhead of 8 bytes, and the x64 CLR has a per-object overhead of 16 bytes.

However, there are minimum sizes of 12 and 24 bytes respectively - it's just that you get the first 4 or 8 bytes "free" when you start storing useful information :)

(See the blog post for more information.)