  Charge exchange (CX) is an important process in shock physics since it
indicates an interaction between downstream ions and ambient neutral hydrogen,
suggesting a presence of a collisionless shock. We present a high-resolution
spectroscopy of an X-ray bright spot in a nearby supernova remnant (SNR), the
Cygnus Loop, with the Reflection Grating Spectrometer (RGS) onboard XMM-Newton.
The target is a compact knotty structure called "southwestern knot (SW-K)"
located at the outer edge of the shell, where the blast wave is likely
interacting with dense surrounding materials. The RGS spectrum of the SW-K
shows details of the line features below ~ 1 keV, where we discover a high
forbidden-to-resonance line ratio of OVII He$\alpha$. The soft-band (10-35 \AA)
spectrum is well explained by a thermal component with a CX X-ray emission
obscured by neutral and ionized absorbers. The presence of the CX X-ray
emission will provide new insights into the shock physics of SNRs. The
high-resolution spectroscopy also reveals that the CNO, Ne and Fe abundances
are truly lower than the solar values (0.2-0.4 solar) at the SW-K region . Our
result gives a clue to solving the previously known "low-abundance problem"
reported from a number of evolved SNRs.
