  We present the X-ray grating spectra of the recurrent nova RS Ophiuchi during
its 2006 outburst, obtained with the XMM-Newton and Chandra observatories. Two
weeks after optical maximum, the X-ray spectrum was hard and dominated by
emission lines of H-like and He-like ions. The X-ray luminosity was 2.4 x 10^36
erg s^-1 in the 0.33-10 keV range. The spectra indicate a collisionally
dominated plasma with a broad range of temperatures. All the lines are
blue-shifted, with the velocity shift increasing with lower ionization state
and longer wavelength. Two weeks later, the spectrum was still dominated by
emission lines, although the line ratios present indicate cooling. During this
observation, a soft X-ray flare occurred in which a new system of higher
velocity emission lines appeared in the soft end of the spectrum. The dominant
component during the third month was the supersoft continuum with the broad
absorption features of a hot white dwarf atmosphere. A preliminary model fit
indicates a white dwarf temperature slightly exceeding 800,000 K. The X-ray
luminosity reached at least 9 x 10^37 erg s^-1 in the 0.2-1 keV range, while
the intrinsic nebular absorption decreased by a factor of five since the first
observation. The spectral fits indicate a massive white dwarf, with a mass of
at least 1.2 M_\odot. Therefore, RS Oph may be an important type Ia supernova
progenitor. We show that the data are consistent with mass loss ending before
day 54 after the outburst, and nuclear burning ending around day 69. A rapid
decay in X-ray luminosity followed after week 10. The X-ray luminosity 5, 7 and
8 months after optical maximum dropped by more than two orders of magnitude.
The spectra do not appear to consistent with emission from an accretion disk.
