Hybrid stars are compact stars that consist of a core of pure quark or mixed hadron-quark matter and outer layers of hadronic (or nuclear) matter. Depending on its features, the hadron-quark phase transition can lead to a separate mass-radius stable branch of hybrid stars and therefore to "twin-star" solutions, i.e. stars with similar masses and different radii. Therefore, a proof of the existence of twin stars by experimental measurements of masses radii and tidal deformabilities may provide a signature for a phase transition inside neutron stars.
In this talk I will show that the data from the gravitational-wave event GW170817 is compatible with the existence of twin stars and could also be interpreted as the merger of a binary system of hybrid stars or of a hybrid star with a neutron star . In addition, combining the 2Msun observations and the information from GW170817 I will set constraints on the parameters that characterise the phase transition, the maximum masses, and the radii of 1.4Msun stars described by equations of state supporting twin-star configurations.
 Montaña, G., Tolós, L., Hanauske, M., Rezzolla, L., Constraining twin stars with GW170817, (Accepted in Phys. Rev. D) arXiv:1811.10929