The cyclotron frequency ratio of $^{ 187}\mathrm{ Os}^{ 29+}$ to $^{ 187}\mathrm{ Re}^{ 29+}$ ions was measured with the Penning-trap mass spectrometer PENTATRAP. The achieved result of $R=1.000\:000\:013\:882(5)$ is to date the most precise such measurement performed on ions. Furthermore, the total binding-energy difference of the 29 missing electrons in Re and Os was calculated by relativistic multiconfiguration methods, yielding the value of $\Delta E = 53.5(10)$ eV. Finally, using the achieved results, the mass difference between neutral $^{ 187}$Re and $^{ 187}$Os, i.e., the $Q$ value of the $\beta^-$ decay of $^{ 187}$Re, is determined to be 2470.9(13) eV.
Categories: nucl-ex