[R-sig-Geo] Drawing circular contours

[Gabor Grothendieck]

I am currently doing the following to draw circles at the
indicated distances from the center.  The center is given
by utm coordinates: utm.e, utm.n .  It does work:

class(im2) # "SpatialGridDataFrame"
image(im2, "ind", col = col$ct)
library(plotrix)
dd <- c(25, 50, 100, 150)
for(i in 1:4) draw.circle(utm.e, utm.n, dd[i], border = "yellow")

However, I would like to do this in a style better suited to sp
and rgdal by defining a function whose contours are circles and
plotting the indicated contours of it.  Also I assume I will get
labelled contours by doing that which I don't currently have.

How do I do that?  I would appreciate specific code, if possible,

Thanks.