Relation between the fixed point eccentricity e_b and the Love number k₂. The blue line shows the fourth order polynomial fit from Batygin et al. (2009). Different eccentricity measurements from Bakos et al. (2009) and Winn et al. (2010) are plotted in black and red, respectively, with solid lines showing the mean value of the measured eccentricity and dashed lines showing the error bars. The black dotted lines indicate the k₂ interval inferred from Batygin et al. (2009) based on the eccentricity measurement from Bakos et al. (2009), in the figure also referred to as old k₂ interval. The gray shaded area marks a region of k₂ values not possible for models of HAT-P-13b. The vertical black solid line (M_core = 0) shows the maximum possible k₂ value we found based on our interior modeling (see Sect. 4 for details). Combining the information of the e_b − k₂-relation from Batygin et al. (2009), the new eccentricity measurement from Winn et al. (2010), and our interior modeling, we find a new k₂ interval ranging from 0.265 − 0.379 (rosy shaded).