Various theoretical models treating the effect of stellar irradiation on planetary envelopes predict the presence of a radius valley, i.e. a bimodal distribution of planet radii, with super-Earths and sub-Neptune planets separated by a valley at around
≈2R⊕
. Such a valley has been observed recently, owing to an improvement in the precision of stellar and therefore planetary radii. Here, we investigate the presence, location, and shape of such a valley using a small sample with highly accurate stellar parameters determined from asteroseismology, which includes 117 planets with a median uncertainty on the radius of 3.3 per cent. We detect a clear bimodal distribution, with super-Earths (
≈1.5R⊕
) and sub-Neptunes (≈2.5 R⊕) separated by a deficiency around
2R⊕
. We furthermore characterize the slope of the valley as a power law R∝Pγ with
γ=−0.09+0.02−0.04
. A negative slope is consistent with models of photoevaporation, but not with the late formation of rocky planets in a gas-poor environment, which would lead to a slope of opposite sign. The exact location of the gap further points to planet cores consisting of a significant fraction of rocky material.