We present a simultaneous analysis of the X1 Σ + and the a3 Σ + electronic ground states of the NaK molecule. Excitation of the [B 1 Π, c3 Σ +]-system made it possible to record fluorescence to rovibrational levels of both ground states simultaneously with a Fourier-transform spectrometer. For the first time high lying levels in the triplet state with v = 17 and v = 18 were seen, the highest possible v = 19 for 23Na-39K was not observed. The joint evaluation of the retrieved data leads to accurate potential energy curves, that describe the experimental data within their experimental uncertainty of 0.005 cm−1. Cold collision properties like scattering length and Feshbach resonance positions are calculated with these potentials and compared to other predictions.