[R-sig-Geo] how to draw a new circle/round polygon on existing plot

[Wouter Steenbeek]

I have a plot with several layers of SpatialPoints and Polygons. They are all projected and in the correct CRS, so my map looks like it should. I now want to draw a circle with a radius x on a part of this map. This should not just be a circle, but be in the correct projection (so that I can change radius x to whatever I desire, and the circle is drawn correctly on scale, and slightly "distorted" dependent on my zoom of the map).

How do I do this? Thanks!

(I understand that I should define a new polygon and "plot(..., add=T)" that to my map. But how do I make such a polygon? I don't quite understand the "polygon" function in the 'graphics' package, if that's even the correct one to use. Seeing an example would help.)