Follow-up on three poorly studied AM CVn stars

We report follow-up observations of three poorly studied AM CVn-type binaries: CRTS CSS150211 J091017–200813, NSV 1440, and SDSS J183131.63+420220.2. Analysing time-series photometry obtained with a range of ground-based facilities as well as with the Transiting Exoplanet Survey Satellite (TESS), we determine the superhump period of CRTS J0910–2008 as $\rm{P_{sh}}=29.700\pm 0.004$ min and the orbital period of NSV 1440 as $\rm{P_{orb}}=36.56\pm 0.03$ min. We also confirm a photometric period of $P=23.026\pm 0.097$ min in SDSS J1831+4202, which is most likely the superhump period. We also report the first optical spectroscopy of CRTS J0910–2008 and NSV 1440 which unambiguously confirms both as AM CVn systems. We briefly discuss the distribution in the Hertzsprung–Russell diagram of the currently known sample of 63 AM CVn stars with known periods and Gaia data.