ElectronicgFactor of Hydrogenlike OxygenO7+16

We present an experimental value for the $g$ factor of the electron bound in hydrogenlike oxygen, which is found to be $g_{\mathrm{e}\mathrm{x}\mathrm{p}\mathrm{t}}=2.0000470254(15)(44)$. The experiment was performed on a single ${}^{16}\mathrm{O}^{7+}$ ion stored in a Penning trap. For the first time, the expected line shape of the $g$-factor resonance is calculated which is essential for minimizing the systematic uncertainties. The measurement agrees within $1.1\sigma$ with the predicted theoretical value $g_{\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{o}\mathrm{r}\mathrm{y}}=2.0000470202(6)$. It represents a stringent test of bound-state quantum electrodynamics to a 0.25% level. Assuming the validity of the underlying theory, a value for the electron mass is obtained: $m_e=0.0005485799096(4)\mathrm{u}$. This value agrees with our earlier determination on ${}^{12}{\mathrm{C}}^{\mathrm{5}\mathrm{+}}$ and allows a combination of both values which is about 4 times more precise than the currently accepted one.