  Temperature-dependent London penetration depth, $\lambda(T)$, was measured in
optimally - doped, $x=$0.35, as-grown ($T_c \approx$25 K,
RRR=$\rho(300K)/\rho(T_c)$=4.5) and annealed ($T_c \approx$35 K, RRR=6.4)
single crystals of SrFe$_2$(As$_{1-x}$P$_{x}$)$_2$ iron - based superconductor.
Annealing increases the RRR and decreases the absolute value of the London
penetration depth from $\lambda(0) = 300 \pm 10$ nm in as-grown sample to
$\lambda(0) = 275 \pm 10$ nm. At low temperatures, $\lambda(T) \sim T$
indicating superconducting gap with line nodes. Analysis of the full -
temperature range superfluid density is consistent with the line nodes, but
differs from the simple single - gap $d-$wave. The observed behavior is very
similar to that of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$, showing that isovalently
substituted pnictides are inherently different from the charge - doped
materials.
