The 1051 s period of the interacting binary white dwarf amcvn

Summary. The close binary system AM CVn consists of two white dwarfs; the lower mass white dwarf, a helium white dwarf, is transferring mass to its higher mass companion. The light curve of AM CVn has a double humped variation with a period of 1051 s that previously was identified with the orbital period of the system. An earlier measurement of the 1051 s period seemed to show that it was increasing rapidly, too rapidly to be easily understood. We have, therefore, remeasured the rate of change of the 1051 s period. We find that the period is changing two orders of magnitude more slowly than previously thought: dP/dt= (- 3.2±0.6) 10- 12 s s- 1• Since the mass losing star in AM CVn is a white dwarf, the orbital period of the system must be increasing. We have found that the 1051 s period is decreasing, and, therefore, it cannot be caused by orbital motion. We reinterpret the 1051 s period as the rotation period of the accreting white dwarf, which must then be magnetized. We have also found a 1011.4 s period in the light curve that may be related to the orbital period, but cannot itself be the orbital period. Finally, we show that the double humped light curve may really be a single humped light curve with a period of 525.5s.