We present the first detection of orbital motion in the cataclysmic variable EX Hydrae based on X-ray data from the Chandra X-ray Observatory .
The large collecting area of the telescope and the high resolution of the HETG spectrometers allow for an unprecedented velocity accuracy of \sim 15 km s ^ { -1 } in the X-ray wavelength regime .
We find an emission line velocity amplitude of 58.2 \pm 3.7 km s ^ { -1 } and infer a white dwarf mass of 0.49 \pm 0.13 ~ { } { M } _ { \odot } , in good agreement with previous studies using optical , ultraviolet , and far ultraviolet data .