  We present the discovery of a very hot gas phase of the Milky Way
circumgalactic medium (CGM) at T $\approx 10^7$ K, using deep XMM-Newton RGS
observations of 1ES 1553+113. The hot gas, coexisting with a warm-hot phase at
T $\approx 10^6$ K is $\alpha-$enhanced, with [O/Fe] = 0.9$^{+0.7}_{-0.3}$,
indicating core-collapse supernovae enrichment. Additionally we find [Ne/O] and
[N/O] = $0.7^{+1.6}_{-0.2}$, such that N/Ne is consistent with solar. Along
with the enrichment by AGB stars and core-collapse supernovae, this indicates
that some oxygen has depleted onto dust and/or transited to cooler gas
phase(s). These results may affect previous baryonic and metallic mass
estimations of the warm-hot and hot CGM from the observations of oxygen
emission and absorption. Our results provide insights on the heating, mixing
and chemical enrichment of the Milky Way CGM, and provide inputs to theoretical
models of galaxy evolution.
