  The hot interstellar medium is an important part of the Galactic ecosystem
and can be effectively characterized through X-ray absorption line
spectroscopy. However, in a study of the hot medium using the accreting neutron
star X-ray binary, Cyg X-2, as a background light source, a mystery came about
when the putatively strong OVII Kalpha line was not detected in Chandra grating
observations, while other normally weaker lines such as OVII Kbeta as well as
OVI and OVIII Kalpha are clearly present (Yao et al. 2009). We have
investigated the grating spectra of Cyg X-2 from 10 XMM-Newton observations, in
search of the missing line. We detect it consistently in nine of these
observations, but the line is absent in the remaining one observation or is
inconsistent with the detection in others at a 4sigma confidence level. This
absence of the line resembles that seen in the Chandra observations. Similarly,
the OVI Kalpha line is found to disappear occasionally, but not in concert with
the variation of the OVII Kalpha line. All these variations are most likely due
to the presence of changing OVII and OVI Kalpha emission lines of Cyg X-2,
which are blurred together with the absorption ones in the X-ray spectra. A
re-examination of the Chandra grating data indeed shows evidence for a narrow
emission line slightly off the OVI Kalpha absorption line. We further show that
narrow NV emission lines with varying centroids and fluxes are present in
far-UV spectra from the Cosmic Origins Spectrograph aboard the Hubble Space
Telescope. These results provide new constraints on the accretion around the
neutron star and on the X-ray-heating of the stellar companion. The
understanding of these physical processes is also important to the fidelity of
using such local X-ray binaries for interstellar absorption line spectroscopy.
