  The recurrent nova (RN) V3890 Sgr was observed during the 7th day after the
onset of its most recent outburst, with the Chandra ACIS-S camera and High
Energy Transmission Gratings (HETG). A rich emission line spectrum was
detected, due to transitions of Fe-L and K-shell ions ranging from neon to
iron. The measured absorbed flux is $\approx 10^{-10}$ erg cm$^{-2}$ s$^{-1}$
in the 1.4-15 Angstrom range (0.77-8.86 keV). The line profiles are asymmetric,
blue-shifted and skewed towards the blue side, as if the ejecta moving towards
us are less absorbed than the receding ones. The full width at half maximum of
most emission lines is 1000-1200 km s$^{-1}$, with some extended blue wings.
The spectrum is thermal and consistent with a plasma in collisional ionization
equilibrium with column density 1.3 $\times 10^{22}$ cm$^{-2}$ and at least two
components at temperatures of about 1 keV and 4 keV, possibly a forward and a
reverse shock, or regions with differently mixed ejecta and red giant wind. The
spectrum is remarkably similar to the symbiotic RNe V745 Sco and RS Oph, but we
cannot distinguish whether the shocks occurred at a distance of few AU from the
red giant, or near the giant's photosphere, in a high density medium containing
only a small mass. The ratios of the flux in lines of aluminum, magnesium and
neon relative to the flux in lines of silicon and iron probably indicate a
carbon-oxygen white dwarf (CO WD).
