  High speed photometry in 2008 shows that the light curve of V842 Cen
possesses a coherent modulation at 56.825 s, with sidebands at 56.598 s and
57.054 s. These have appeared since this nova remnant was observed in 2000 and
2002. We deduce that the dominant signal is the rotation period of the white
dwarf primary and the sidebands are caused by reprocessing from a surface
moving with an orbital period of 3.94 h. Thus V842 Cen is an intermediate polar
(IP) of the DQ Herculis subclass, is the fastest rotating white dwarf among the
IPs and is the third fastest known in a cataclysmic variable. As in other IPs
we see no dwarf nova oscillations, but there are often quasi-periodic
oscillations in the range 350 - 1500 s. There is a strong brightness modulation
with a period of 3.78 h, which we attribute to negative superhumps, and there
is an even stronger signal at 2.886 h which is of unknown origin but is
probably a further example of that seen in GW Lib and some other systems. We
used the Swift satellite to observe V842 Cen in the ultra-violet and in X-rays,
although no periodic modulation was detected in the short observations. The
X-ray luminosity of this object appears to be much lower than that of other IPs
in which the accretion region is directly visible.
