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{\it Kepler} satellite photometry and phase-resolved spectroscopy of theultracompact AM CVn type binary SDSS J190817.07+394036.4 are presented. Theaverage 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 spectraand the Doppler tomograms reveal an S-wave in emission in the core of the He I4471 \AA\,absorption line at a period of $P_{\rm orb}=1085.7\pm2.8$\,secidentifying this as the orbital period of the system. The Si II, Mg II and thecore of some He I lines show an S-wave in absorption with a phase offset of$170\pm15^\circ$ compared to the S-wave in emission. The N II, Si III and somehelium lines do not show any phase variability at all. The spectroscopicorbital period is in excellent agreement with a period at $P_{\rmorb}=1085.108(9)$\,sec detected in the three year {\it Kepler} lightcurve. AFourier analysis of the Q6 to Q17 short cadence data obtained by {\it Kepler}revealed a large number of frequencies above the noise level where the majorityshows a large variability in frequency and amplitude. In an O-C analysis wemeasured a $\vert\dot{P}\vert\sim1.0\,$x$\,10^{-8}\,$s\,s$^{-1}$ for some ofthe strongest variations and set a limit for the orbital period to be$\vert\dot{P}\vert<10^{-10}$s\,s$^{-1}$. The shape of the phase foldedlightcurve on the orbital period indicates the motion of the bright spot.Models of the system were constructed to see whether the phases of the radialvelocity curves and the lightcurve variation can be combined to a coherentpicture. However, from the measured phases neither the absorption nor theemission can be explained to originate in the bright spot.

Phase resolved spectroscopy and Kepler photometry of the ultracompact AM CVn binary SDSS J190817.07+394036.4