Mass-radius relationship for different bulk compositions of small planets. We overplot different compositions for solid planets in between pure iron and pure water. The black dashed line corresponds to the models of Baraffe et al. (2008) for gaseous planets with heavy material enrichments of 0.9 and an age of 5 Gyr. We superimposed the known planets in this mass-radius range where the greyscale depends on the precision of the mass and radius. We mark the position of the planets HD 106315b and HD 106315c, as derived by us.