  For the He/N nova M31N 2007-12b, we analyzed XMM-Newton EPIC and Chandra
HRC-I observations of our monitoring program performed at intervals of ten days
and added results of a XMM-Newton target of opportunity observation and Swift
XRT observations. The supersoft source (SSS) emission started between 21 and 30
d after the optical outburst and ended between 60 and 120 d after outburst,
making M31N 2007-12b one of the few novae with the shortest SSS phase known.
The X-ray spectrum was supersoft and can be fitted with a white dwarf (WD)
atmosphere model with solar abundances absorbed by the Galactic foreground. The
temperature of the WD atmosphere seems to increase at the beginning of the SSS
phase from ~70 to ~80 eV. The luminosity of M31N 2007-12b during maximum was at
the Eddington limit of a massive WD and dropped by ~30% in the observation 60 d
after outburst. The radius of the emission region is ~6x10^8 cm. In the four
bright state observations, we detected a stable 1110 s pulsation, which we
interpret as the WD rotation period. In addition, we detect dips in three
observations that might represent a 4.9 h or 9.8 h binary period of the system.
Nova envelope models with <50% mixing between solar-like accreted material and
the degenerate core of the WD can be used to describe the data. We derive a WD
mass of 1.2 Msun, as well as an ejected and burned mass of 2.0x10^{-6} Msun}
and 0.2x10^{-6} Msun, respectively. The observed periodicities indicate that
nova M31N 2007-12b erupted in an intermediate polar (IP) system. The WD
photospheric radius seems to be larger than expected for a non-magnetic WD but
in the range for magnetic WDs in an IP system. (abridged)
