We consider the case of very low reheating scenarios ( T _ { RH } \sim \mathcal { O } ( { MeV } ) ) with a better calculation of the production of the relic neutrino background ( with three-flavor oscillations ) .
At 95 % confidence level , a lower bound on the reheating temperature T _ { RH } > 4.1 MeV is obtained from Big Bang Nucleosynthesis , while T _ { RH } > 4.3 MeV from Planck data for very light ( \sum m _ { i } = 0.06 \mathrm { eV } ) neutrinos .
If neutrino masses are allowed to vary , Planck data yield T _ { RH } > 4.7 MeV , the most stringent bound on the reheating temperature to date .
Neutrino masses as large as 1 eV are possible for very low reheating temperatures .