C59N magnetic fullerenes inside single-wall carbon nanotubes (SWCNTs) are used to probe the density of states (DOS) on the host tubes using electron spin resonance (ESR). The C59N radicals are separated by C60 fullerenes to prevent dimerization and C59N-C60 heterodimers are formed at low temperatures. The electron spin-lattice relaxation time, T1, of the heterodimers is deduced from the homogeneous ESR line-width. The analysis is supported by saturation ESR studies. The inverse of the heterodimer T1 follows a linear behavior in the 20-300 K temperature range, the so-called Korringa law, evidencing a metallic DOS on all tubes in a bundle.