Given two circles A(R), B(r) such that their centers are at a distance [ d < R+r ], there is an ellipse [ (x/a)²+(y/b)²=1, look at Ellipse.html ] whose points C are at equal distances from the two circles. Its major axis (a) is the sum of the radii of the circles, (c) is |AB|/2 and b² = c²-a².

Switch to the [Select on contour] tool (Ctrl+2) catch and move C. The circles are controled using the [Selection] tool (Ctrl+1) and the points A, L, B and K. Look at the file ConicsFromCir.html for the general case.