describe photon diffusion, we may use a simple one-zone approximation (Arnett [@arn79]), ignoring the contribution of radioactive decays to the bolometric light curve: $${d \ln L_{bol} \over d t} = - {1 \over t_{dif}} \; , \,
   t_{dif} = {\pi \over 3}{R_d \over c} \tau_d \; ,
   \label{eq:lboldif}$$ where $t_{dif}$ is the diffusion time, $R_d$ the characteristic radius of the diffusion region, and $\tau_d$ the optical depth of this region.

Evidently, it is possible to estimate the diffusion time from both the decline of the bolometric light curve and the physical properties of the diffusion region. The decline of the luminosity shoulder between day 94 and day 116 measures a diffusion time of $\approx 35$ days (Fig. \[fig:snphs\]). On the other hand, the radius of the diffusion region of $\approx 1.2 \times 10^{15}$ cm and its optical depth of $\approx 63$ evaluated at day 105 give a diffusion time of $\approx 30$ days. These estimates are consistent with the above description of photon diffusion (\[eq:lboldif\]). Thus, the luminosity shoulder at the phase of radiative diffusion cooling is approximated well by the simple model with the constant diffusion time. Note that a transition from the constant diffusion time to the variable 