  The formation of a strange or hybrid star from a neutron star progenitor is
believed to occur when the central stellar density exceeds a critical value. If
the transition from hadron to quark matter is of first order, the event has to
release a huge amount of energy in a very short time and we would be able to
observe the phenomenon even if it is at cosmological distance far from us; most
likely, such violent quark deconfinement would be associated with at least a
fraction of the observed gamma ray bursts. If we allow for temporal variations
of fundamental constants like $\Lambda_{QCD}$ or $G_N$, we can expect that
neutron stars with an initial central density just below the critical value can
enter into the region where strange or hybrid stars are the true ground state.
  From the observed rate of long gamma ray bursts, we are able to deduce the
constraint $\dot{G}_N/G_N \lesssim 10^{-17} {\rm yr^{-1}}$, which is about 5
orders of magnitude more stringent than the strongest previous bounds on a
possible increasing $G_N$.
