For the case when a and b =0, all values of r will equal zero. Thus we get what we might refer to as a point circle.
If we look at the relationship between the value of a and b, and the distance from the origin to the end of a leaf (call it the leaf radius), we should notice that the length of each leaf is a+b, or twice the value of either a or b. This should make good sense as the largest value that the cosine function can achieve is 1, thus the largest value of r would have to be (a+b)(1). Since a and b are equal in this case, this would also translate to (a+a)(1)=2a, or (b+b)(1)=2b.