  With the first detection of gravitational waves from a binary system of
neutron stars, GW170817, a new window was opened to study the properties of
matter at and above nuclear-saturation density. Reaching densities a few times
that of nuclear matter and temperatures up to $100\,\rm{MeV}$, such mergers
also represent potential sites for a phase transition (PT) from confined
hadronic matter to deconfined quark matter. While the lack of a postmerger
signal in GW170817 has prevented us from assessing experimentally this
scenario, two theoretical studies have explored the postmerger
gravitational-wave signatures of PTs in mergers of binary systems of neutron
stars. We here extend and complete the picture by presenting a novel signature
of the occurrence of a PT. More specifically, using fully general-relativistic
hydrodynamic simulations and employing a suitably constructed equation of state
that includes a PT, we present the occurrence of a "delayed PT", i.e. a PT that
develops only some time after the merger and produces a metastable object with
a quark-matter core, i.e. a hypermassive hybrid star. Because in this scenario,
the postmerger signal exhibits two distinct fundamental gravitational-wave
frequencies -- before and after the PT -- the associated signature promises to
be the strongest and cleanest among those considered so far, and one of the
best signatures of the production of quark matter in the present Universe.
