  We study the cooling behaviour of an isolated strange quark star, using an
equation of state derived from perturbative QCD up to second order in strong
coupling constant, and we compare it with that of a neutron star. After an
initial rapid cooling, a quark star may undergo the QCD phase transition to
become a neutron star. We propose several signatures for such a scenario: a
large amount of energy can be released due to latent heat, a long duration
$\gamma$-ray source, and a second neutrino burst after a supernova explosion.
