X-ray grating spectra have opened a new window on the nova physics. Highsignal-to-noise spectra have been obtained for 12 novae after the outburst inthe last 13 years with the Chandra and XMM-Newton gratings. They offer the only way to probe the temperature, effective gravity andchemical composition of the hydrogen burning white dwarf before it turns off.These spectra also allow an analysis of the ejecta, which can be photoionizedby the hot white dwarf, but more often seem to undergo collisional ionization. The long observations required for the gratings have revealed semi-regularand irregular variability in X-ray flux and spectra. Large short termvariability is especially evident in the first weeks after the ejecta havebecome transparent to the central supersoft X-ray source. Thanks to Chandra andXMM-Newton, we have discovered violent phenomena in the ejecta, discrete shellejection, and clumpy emission regions. As expected, we have also unveiled the white dwarf characteristics. The peakwhite dwarf effective temperature in the targets of our samples varies between~400,000 K and over a million K, with most cases closer to the upper end,although for two novae only upper limits around 200,000 K were obtained. Acombination of results from different X-ray satellites and instruments,including Swift and ROSAT, shows that the shorter is the supersoft X-ray phase,the lower is the white dwarf peak effective temperature, consistently withtheoretical predictions. The peak temperature is also inversely correlated witht(2) the time for a decay by 2 mag in optical. I strongly advocate the use ofwhite dwarf atmospheric models to obtain a coherent physical picture of thehydrogen burning process and of the surrounding ejecta.
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