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 He i 4471 Å absorption line at a period of P _ { orb } = 1085.7 \pm 2.8 sec identifying this as the orbital period of the system .
The Si ii , Mg ii and the core of some He i lines show an S-wave in absorption with a phase offset of 170 \pm 15 ^ { \circ } compared to the S-wave in emission .
The N ii , Si iii and some helium lines do not show any phase variability at all .
The spectroscopic orbital period is in excellent agreement with a period at P _ { orb } = 1085.108 ( 9 ) sec detected in the three year Kepler lightcurve .
A Fourier analysis of the Q6 to 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 O-C analysis we measured a | \dot { P } | \sim 1.0 x 10 ^ { -8 } s s ^ { -1 } for some of the strongest variations and set a limit for the orbital period to be | \dot { P } | < 10 ^ { -10 } s s ^ { -1 } .
The shape of the phase folded lightcurve 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 lightcurve 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 .