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VX Fornacis

VX For is a southern member of the WZ Sge class of dwarf novae.  VX For went into outburst in mid-September 2009, remaining active for around 50 days.  Its outburst, like those of other WZ Sge stars, was characterized by a large amplitude, long-duration outburst of 7 magnitudes that included a number of rapid fadings and rebrightenings known as "echo outbursts".  VX For also exhibited one of the other hallmarks of this class, a phenomenon known as "superhumps".  Superhumps are periodicities offset from the orbital period caused by precession of the hotspot of the accretion disk.  It takes the disk of these stars a few days to precess, and so the superhumps are a beat period of the orbital period and the precession period.  Interestingly, the variations seen in this outburst of VX For were unlike those of other WZ Sge stars in that the superhump period didn't wander much during the outburst.  The reason for this isn't yet known, although its possible that observers caught VX For later in its outburst than thought, and all of the period variations occurred earlier.

AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484