V4633 Sgr : a probable second asynchronous polar classical nova

ABSTRACT Photometric observationsof V4633Sgr (Nova Sagittarii 1998)during 1998-2005revealthe presence of a stable photometric periodicity at P 1 = 180.8 min which is probablythe orbital period of the underlying binary system. A second period was present inthe light curve of the object for six years. Shortly after the nova eruption it wasmeasured as P 2 = 185.6 min. It has decreased monotonically in the following fewyears reaching the value P 2 = 183.9 min in 2003. In 2004 it was no longer detectable.We suggest that the second periodicity is the spin of the magnetic white dwarf ofthis system that rotates nearly synchronously with the orbital revolution. Accordingto our interpretation, the post-eruption evolution of Nova V4633 Sgr follows a tracksimilar to the one taken by V1500 Cyg (Nova Cygni 1975) after that nova eruption,on a somewhat longer time scale. The asynchronism is probably the result of the novaoutburst that lead to a considerable expansion of the white dwarf’s photosphere. Theincrease in the moment of inertia of the star was associated with a correspondingdecrease in its spin rate. Our observations have followed the spinning up of the whitedwarfresulting from the contraction of its outer envelope as the star is slowly retuningtoitspre-outburststate.Itisthusthesecondknownasynchronouspolarclassicalnova.Key words: binaries: close – stars: individual: V4633 Sgr – novae, cataclysmic vari-ables.