Getting position of a coordinate on the surface of a sphere

So I want to create a object B close to another object A, but I'm trying to avoid that A covers B:

I have thinking how to achieve that. Take a look of the following drawing:

I have been thinking in a solution in a math - geometry way before searching for some Unity3D function to solve the problem. I have realise A and B would be on the same surface if the camera is within a sphere with radius c-A. So I guess the solution can be related to get a point at a B at a distance A-B from A on the surface of the sphere with radius c-A. Does it has sense? Any other idea? How to do it with maths and Unity?