From 1973 to 1987 the University of Tübingen and MPE performed thirteen balloon flights with hard X-ray detectors (High Energy X-ray Experiment - HEXE). One of the results was the discovery of cyclotron lines in the X-ray spectrum of the mass accreting neutron star Hercules X-1 which for the first time allowed the spectroscopic measurement of a neutron star magnetic field B ~ 4 × 10¹² Gauß.