We explore the implications of the recent radius determination of PSR $\mathrm{J}0740+6620$ by the NICER experiment, combined with the neutron skin measurement by the PREX-II experiment and the associated inference of the slope of symmetry energy, for the structure of hybrid stars with a strong first-order phase transition from nucleonic to quark matter. We combine a covariant density-functional nucleonic equation of state (EOS) with a constant-speed-of-sound EOS for quark matter. We show that the radius and tidal deformability ranges obtained from GW170817 can be reconciled with the implication of the PREX-II experiment if there is a phase transition to quark matter in the low-mass compact star. In the high-mass segment, the EOS needs to be stiff to comply with the large-radius inference for PSR $\mathrm{J}0740+6620$ and $\mathrm{J}0030+0451$ with masses $M\simeq 2M_{\odot }$ and $M\simeq 1.4M_{\odot }$. We show that twin stars are not excluded, but the mass and radius ranges (with $M\ge M_{\odot }$) are restricted to narrow domains $\mathrm{\Delta }{M}_{\mathrm{twin}}\lesssim 0.05M_{\odot }$ and $\mathrm{\Delta }{R}_{\mathrm{twin}}\sim 1.0\mathrm{km}$. We also show that the existence of twin configurations is compatible with the light companion in the GW190814 event being a hybrid star in the case of values of the sound-speed square $s=0.6$ and $s=1/3$.

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• Nuclear matter in neutron stars
• Neutron stars & pulsars