  A Zeeman-insensitive optical clock atomic transition is engineered when
nuclear spins are dressed by a non resonant radio-frequency field. For
fermionic species as $^{87}$Sr, $^{171}$Yb, and $^{199}$Hg, particular ratios
between the radiofrequency driving amplitude and frequency lead to "magic"
magnetic values where a net cancelation of the Zeeman clock shift and a
complete reduction of first order magnetic variations are produced within a
relative uncertainty below the $10^{-18}$ level. An Autler-Townes continued
fraction describing a semi-classical radio-frequency dressed spin is
numerically computed and compared to an analytical quantum description
including higher order magnetic field corrections to the dressed energies.
