Summary: We analyze the generation of seed magnetic fields during de Sitter inflation considering a noninvariant conformal term in the electromagnetic Lagrangian of the form \(-\frac 1 4 I(\phi)F_{\mu \nu}\widetilde F^{\mu \nu }\), where \(I(\phi )\) is a pseudoscalar function of a nontrivial background field \(\phi \). In particular, we consider a toy model that could be realized owing to the coupling between the photon and either a (tachyonic) massive pseudoscalar field or a massless pseudoscalar field nonminimally coupled to gravity, where I follows a simple power law behavior \(I(k,\eta ) = g/(-k\eta ) ^{\beta }\) during inflation, while it is negligibly small subsequently. Here, \(g\) is a positive dimensionless constant, \(k\) the wave number, \(\eta \) the conformal time, and \(\beta \) a real positive number. We find that only when \(\beta = 1\) and \(0.1 \lesssim g \lesssim 2\) can astrophysically interesting fields be produced as excitation of the vacuum, and that they are maximally helical.