LAMOST spectra and photometric behaviour of four AM CVn binaries

Abstract

We present a study of the LAMOST spectra and public photometric data for four AM CVn binaries with orbital periods ranging from 17 to 41 min. The prototypical star AM CVn displays short-term brightness fluctuations, which most likely arise from its superhump modulation. The superhump period of PTF1J0719+4858 is identified to be 1659.85(13) s, slightly longer than its orbital period. We find a short-lived dip in the phase-folded light curve centered on a phase of ∼ 0.5 during the 2008 March outburst of YZ LMi. This dip can be interpreted as the obscuration of the accretion flow or inner disc. The superhump period of V493 Gem has a positive period derivative during the middle stage, which can be seen in most SU UMa-type cataclysmic variables (CVs). By applying the growing superhump method we determined its mass ratio to be q=0.0315(24). The quiescent spectra of both PTF1J0719+4858 and V493 Gem show clear metal lines and reveal a possible white dwarf donor. We also discussed the theoretical values of the donor mass and the mass transfer rate of these two systems.