  The spin-down of a neutron star, e.g. due to magneto-dipole losses, results
in compression of the stellar matter and induces nuclear reactions at phase
transitions between different nuclear species in the crust. We show that this
mechanism is effective in heating recycled pulsars, in which the previous
accretion process has already been compressing the crust, so it is not in
nuclear equilibrium. We calculate the corresponding emissivity and confront it
with available observations, showing that it might account for the likely
thermal ultraviolet emission of PSR J0437-4715.
