 The aim of the treatment is to draw lines of ablation in specific places in the heart. Unfortunately, a lot of patients almost 50 get AF again after this treatment and most of these patients have a second treatment performed. It is usually found at this second treatment that gaps have developed in the lines of ablation that were drawn the first time around. The investigators think that electively doing a second treatment to close these gaps a couple of months after the first treatment may mean that fewer of these patients will get AF again in the future. The investigators also want to find out what factors make a line of ablation less likely to develop gaps. 