Example (ZTF J0054+1429) of how we determined the magnetic field strength from the spectra. The black line shows the spectrum, the red line the best-fit red dwarf template, and in grey the difference. The blue vertical lines show the cyclotron harmonic wavelengths for the selected magnetic field strength (the numbers next to the lines indicate which harmonic). We used an interactive version of this figure where the B-field strength can be set with a slider to quickly check at which wavelengths the different harmonics should be located.