tokens: and quasi-bolometric light curves of the photospheric phase and 56 Ni tail. The sample is heterogeneous, ranging over a factor of 30 in plateau luminosity and 56 Ni mass, and over a range of 2.5 mag/100 days in decline rate. Namely, the sample includes luminous, intermediate, and low-luminosity SNe II as well as SNe that were classified as II-P and as II-L. We analyze this sample, calculating the new observables that we derive here, and find the following results. 1. 56 Ni contribution is significant in most of the sample. In many SNe, it is the source of about 30% of the time-weighted integrated luminosity during the photospheric phase, but there is a single SN (2009ib) in the sample where 56 Ni contributes 72%, and several SNe where it powers about 10%. 2. 56 Ni contribution can possibly flatten and/or extend the photospheric light curve. We find that if flattening were the only effect, then SNe that are observed to have a flat plateau would have shown a decline rate of about 1 mag/100 days in the absence of 56 Ni. If, on the other hand, the only effect of 56 Ni were to extend the plateau, then without 56 Ni, the photospheric phase would have been typically shorter by a factor of about 1.15–1.35 and in the most extreme cases by almost a factor of two. 3. 56 Ni contribution cannot explain the entire range of observed decline rates, which is mostly an intrinsic property of the cooling envelope. Several independent lines of evidence suggest that the dominant effect of 56 Ni is flattening of the light curve. This includes a significant anti-correlation between the level of 56 Ni contribution and the decline rate (i.e., 56 Ni is more important in SNe with a flatter plateau). This is expected if 56 Ni is mixed throughout a significant fraction of the envelope (as suggested by Bersten et al. 2011 for SN 1999em). With this interpretation, the picture that arises from our results is that flat plateaus are not a generic feature of cooling envelope emission. Instead, many observed plateaus include a significant contribution of 56 Ni and without it, many of the SNe II-P would have shown a decline rate of up to 1 mag/100 days. Note that while not being expected a priori, there is no need for a “conspiracy,” so the combination of cooling envelope and 56 Ni emission would result in plateaus. First, SNe II exhibit a range of decline rates, thus in some cases the combined emission produces a plateau and in others it does not. Second, it seems that even without 56 Ni, the cooling envelope emission often shows a moderate decline rate where the luminosity drops by less than a factor of two over 100 days. In these cases the contribution of 56 Ni is enough to make the light curve even flatter. The coincidence in SNe II seems to be the fact that the contribution of the cooling envelope and 56 Ni-powered emission is comparable over a wide range of light curve luminosities. This is a result of the well known tight correlation between the luminosity and 56 Ni
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