Nodal superconductivity in isovalently substituted SrFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ pnictide superconductor at the optimal doping, $x=$0.35

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)\mathrm{/}\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.