If a phase transition is allowed to take place in the core of a compact star, a new stable branch of equilibrium configurations can appear, providing solutions with the same mass as the purely hadronic branch and hence giving rise to "twin-star" configurations. We perform an extensive analysis of the features of the phase transition leading to twin-star configurations and, at the same time, fulfilling the constraints coming from the maximum mass of 2 M and the information following gravitational-wave event GW170817. In particular, we use a general equation of state for the neutron-star matter that parametrizes the hadron-quark phase transition between the model describing the hadronic phase and a constant speed of sound for the quark phase. We find that the largest number of twin-star solutions has masses in the neutron-star branch that are in the range 1-2 M and maximum masses 2 M in the twin-star branch. The analysis of the masses, radii and tidal deformabilities also reveals that when twin stars appear, the tidal deformability shows two distinct branches with the same mass, thus differing considerably from the behaviour expected for normal neutron stars. In addition, we find that the data from GW170817 is compatible with the existence of hybrid stars and could also be interpreted as produced by the merger of a binary system of hybrid stars or of a hybrid star with a neutron star. Indeed, with the use of a well-established hadronic EOS the presence of a hybrid star in the inspiral phase could be revealed if future gravitational-wave detections measure chirp masses M 1.2 M and tidal deformabilities of Λ1.4 400 for 1.4 M stars. Finally, combining all observational information available so far, we set constraints on the parameters that characterise the phase transition, the maximum masses, and the radii of 1.4 M stars described by equations of state leading to twin-star configurations.

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We explore the possibility that some of the five narrow Ωc resonances recently observed at LHCb could correspond to pentaquark states, structured as meson-baryon bound states or molecules. The interaction of the low-lying pseudoscalar mesons with the ground-state baryons in the charm +1, strangeness −2 and isospin 0 sector is built from t-channel vector meson exchange, using effective Lagrangians. The resulting s-wave coupled-channel unitarized amplitudes show the presence of two structures with similar masses and widths to those of the observed Ωc(3050) 0 and Ωc(3090) 0 . The identification of these resonances with the meson-baryon bound states found in this work would also imply assigning the values 1/2 − for their spin-parity. An experimental determination of the spin-parity of the Ωc(3090) 0 would help in disentangling its structure, as the quark-based models predict its spin-parity to be either 3/2 − or 5/2 − .

The open-charm Euclidean correlators have been computed for the first time using the thermal spectral functions extracted from a finite-temperature self-consistent unitarized approach based on a chiral effective field theory that implements heavy-quark spin symmetry. The inclusion of the full-energy dependent open-charm spectral functions in the calculation of the Euclidean correlators leads to a similar behaviour as the one obtained in lattice QCD for temperatures well below the transition deconfinement temperature. The discrepancies at temperatures close or above the transition deconfinement temperature could indicate that higherenergy states, that are not present in the open-charm spectral functions, become relevant for a quantitative description of the lattice QCD correlators at those temperatures. In fact, we find that the inclusion of a continuum of scattering states improves the comparison at small Euclidean times, whereas differences still arise for large times.