2015MNRAS.453..483K

Kepler satellite photometry and phase-resolved spectroscopy of the ultracompact AM CVn type binary SDSS J190817.07+394036.4 are presented. The average spectra reveal a variety of weak metal lines of different species, including silicon, sulphur and magnesium as well as many lines of nitrogen, beside the strong absorption lines of neutral helium. The phase-folded spectra and the Doppler tomograms reveal an S-wave in emission in the core of the Hei 4471 Å absorption line at a period of Porb = 1085.7±2.8 s identifying this as the orbital period of the system. The Siii, MgII and the core of some Hei lines show an S-wave in absorption with a phase offset of 170°±15° compared to the S-wave in emission. The Nii, SIIII and some helium lines do not show any phase variability at all. The spectroscopic orbital period is in excellent agreement with a period at Porb = 1085.108(9) s detected in the 3 yr Kepler light curve. A Fourier analysis of the Q6-Q17 short-cadence data obtained by Kepler revealed a large number of frequencies above the noise level where the majority shows a large variability in frequency and amplitude. In an Observed-minus-computed analysis, we measured a |{dot}P|∼1.0 x10^–8s/s for some of the strongest variations and set a limit for the orbital period to be |{dot}P|<10^-10 s/s. The shape of the phase-folded light curve on the orbital period indicates the motion of the bright-spot. Models of the system were constructed to see whether the phases of the radial velocity curves and the light-curve variation can be combined to a coherent picture. However, from the measured phases neither the absorption nor the emission can be explained to originate in the bright-spot.