Draw a circle on a filled.contour() plot

Hi all,

I'm trying to use symbols() to draw a circle of a given radius at a  
given position onto a filled.contour() plot. The commands I issue are:

filled.contour(y,x,z,levels=seq 
(0.02,1.0,len=50),color.palette=colorRampPalette(c 
("blue","yellow","red")),title=title(main="",xlab="",ylab=""))
symbols(0.62,0.0,circles=c(2.5),add=TRUE,inches=FALSE

As can be seen on the resulting PDF file (http://www.eleves.ens.fr/ 
home/coudert/hist_2d.pdf), the circle appears to have the right  
radius but its center is not the correct one [0.62, 0]. What am I  
missing here?

Thanks for your help,
FX

PS: complete input (5 lines), data file for the contour plot and PDF  
result on my system can be found at:
Data file: http://www.eleves.ens.fr/home/coudert/hist_2d
R script: http://www.eleves.ens.fr/home/coudert/hist_2d.R
PDF result: http://www.eleves.ens.fr/home/coudert/hist_2d.pdf