2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/15278The occurence of a quark matter core in rotating compact stars has been investigated within general relativity as a function of both the rotational frequency and the total baryon number. We demonstrate that the deviation of the braking index from $n=3$ signals not only the occurence but also the size of a quark matter core in a pulsar. We suggest that in systems with mass accretion onto a rapidly rotating compact star a spin-down to spin-up transition might signal a deconfinement transition in its interior.2 pages, 2 eps figures embedded, to be published in Proceedings of IAU Coll. 177: "Pulsar Astronomy - 2000 and Beyond", ASP Conf. SeriesAstrophysicstext