Upper panel: mass-radius relation for low-mass ultra-short planets. K2-106 b, c are the red filled symbols obtained using the stellar parameters derived by us. The open red symbol shows the same, but for the parameters given by Adams et al. (2017). The blue symbols are all other known ultra-short period planets. Lower panel: detailed position of K2-106 b, c using models calculated by us. The iron core contains about 80% of the mass of the planet.