Born-Oppenheimer Approximation for a Simple Renormalizable System

We discuss a simple singular system in two dimension, two heavy particles interacting with a light particle via an attractive contact interaction. Although intuitively clear the actual application of the Born-Oppenheimer approximation to this problem is quite subtle. Nevertheless, with due care we show that this can be done, as a result we calculate the leading term of the Born-Oppenheimer approximation and indicate how to get the higher order corrections.