Sgr – a probable second asynchronous polar classical nova

Photometric observations of V4633 Sgr (Nova Sagittarii 1998) during 1998-2005 reveal the presence of a stable photometric periodicity at P1 = 180.8 min which is probably the orbital period of the underlying binary system. A second period was present in the light curve of the object for six years. Shortly after the nova eruption it was measured as P2 = 185.6 min. It has decreased monotonically in the following few years reaching the value P2 = 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 of this system that rotates nearly synchronously with the orbital revolution. According to our interpretation, the post-eruption evolution of Nova V4633 Sgr follows a track similar 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 nova outburst that lead to a considerable expansion of the white dwarf’s photosphere. The increase in the moment of inertia of the star was associated with a corresponding decrease in its spin rate. Our observations have followed the spinning up of the white dwarf resulting from the contraction of its outer envelope as the star is slowly retuning to its pre-outburst state. It is thus the second known asynchronous polar classical nova.