Limits on possible magnetic fields at nucleosynthesis time

In this paper we discuss limits on magnetic fields that could have been present at nucleosynthesis time. We considered several effects that could be relevant modifing light elements relic abundances. They include: changes in reaction rates, mass shifts due to strong and electromagnetic interactions, variation of the expansion rate of the Universe due to both the magnetic field energy density and the increasing of the electrons density in overcritical magnetic fields. We find that the latter is the main effect. It was not taken into account in previous calculations. The allowed field intensity at the end of nucleosynthesis ($T =1\times 10^9\;^o{\rm K}$) is $B \le 3\times 10^{10}$ Gauss.