  We show that the zeroth principle of thermodynamics applies to aging
quasistationary states of long-range interacting $N$-body Hamiltonian systems.
We also discuss the measurability of the temperature in these
out-of-equilibrium states using a {\it short-range} interacting thermometer. As
many connections are already established between such quasistationary states
and nonextensive statistical mechanics, our results are the first evidence that
such basic concepts apply to systems that the nonextensive formalism aims to
describe.
