  As a neutron star spins down, the gradual decrease of the centrifugal force
produces a progressive increase of the density of any given fluid element in
its interior. Since the ``chemical'' (or ``beta'') equilibrium state is
determined by the local density, this process leads to a chemical imbalance
quantified by a chemical potential difference, e.g.,
\delta\mu=\mu_n-\mu_p-\mu_e, where n, p, and e denote neutrons, protons, and
electrons. In the presence of superfluid energy gaps, in this case \Delta_n and
\Delta_p, reactions are strongly inhibited as long as both \delta\mu and kT are
much smaller than the gaps. Thus, no restoring mechanism is available, and the
imbalance will grow unimpeded until
\delta\mu=\delta\mu_{thr}=\Delta_n+\Delta_p. At this threshold, the reaction
rate increases dramatically, preventing further growth of \delta\mu, and
converting the excess chemical energy into heat. The thermal luminosity
resulting from this ``rotochemical heating'' process is $L\sim 2\times
10^{-4}(\delta\mu_{thr}/0.1\MeV)\dot E_{rot}$, similar to the typical x-ray
luminosity of pulsars with spin-down power \dot E_{rot}. The threshold
imbalance, and therefore the luminous stage, are only reached by millisecond
pulsars. A preliminary study of eleven millisecond pulsars with reported ROSAT
observations shows that the latter can already be used to start constraining
superfluid energy gaps in the theoretically interesting range, ~ 0.1 - 1 MeV.
