We analysed archival spectroscopic data of AM CVn taken with the William Herschel Telescope in 1996. In the literature two orbital periods for AM CVn are proposed. A clear S-wave in the HeI 4471, 4387 and 4143 \AA lines is revealed when the spectra are folded on the 1029 s period. No signature of this S-wave is seen when folded on 1051 s. Doppler tomography of the line profiles shows a clear signature of the hotspot. Using this we can constrain the value of K_2 to lie between 210 and 280 km/s. Our work confirms the binary nature of AM CVn beyond any doubt, establishes 1028.73 s as the true orbital period and supports the interpretation of AM CVn as a permanent superhump system.
@article{arxiv.astro-ph/0104220,
title = {Spectroscopic evidence for the binary nature of AM CVn},
author = {G. Nelemans and D. Steeghs and P. J. Groot},
journal= {arXiv preprint arXiv:astro-ph/0104220},
year = {2009}
}