  In several iron-arsenide superconductors there is strong evidence for a fully
gapped superconducting state consistent with either a conventional s-wave
symmetry or an unusual $s_\pm$ state where there the gap changes sign between
the electron and hole Fermi surface sheets. Here we report measurements of the
penetration depth $\lambda(T)$ in very clean samples of the related
iron-phosphide superconductor, LaFePO, at temperatures down to $\sim$ 100 mK.
We find that $\lambda(T)$ varies almost perfectly linearly with $T$ strongly
suggesting the presence of gap nodes in this compound. Taken together with
other data, this suggests the gap function may not be generic to all pnictide
superconductors.
