AAVSO: American Association of Variable Star Observers

RU Cam: The Reluctant Cepheid Revisited

Volume 49 number 1 (2021)

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John R. Percy
Department of Astronomy and Astrophysics, and Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada; john.percy@utoronto.ca


Skilled amateur astronomers can still make significant contributions to variable star research, even in this age of massive automated sky surveys. Among other things, they can identify and/or observe stars with unusual properties or behavior. RU Cam, a 22-day carbon-rich Population II Cepheid (CW variable), is one such star. In 1965, Serge Demers and Don Fernie discovered that it had abruptly decreased in full amplitude from 1.0 to 0.1 magnitude. It was subsequently observed intensively until the 1990s, especially at the Konkoly Observatory, and this enabled theoretical discussions about the possible nature of the star’s pulsation. The cause of the amplitude decrease was and still is not clear. Observations have been more sporadic since the 1990s. There is some AAVSO V photometry, and sparse AAVSO visual photometry from before the amplitude decrease to the present. More recently, RU Cam was observed by the All-Sky Automated Survey for Supernovae (ASAS-SN) from 2014 to 2018, and by the Transiting Exoplanet Survey Satellite (TESS) over two pulsation cycles. In this paper, I analyze the ASAS-SN data and the AAVSO data for possible changes in the period and the amplitude. The period has remained more-or-less stable at 22 ± 1 days and, since 1965, the full amplitude has continued to vary from less than 0.1 to about 0.3 on a time scale of hundreds of days (tens of pulsation periods), reminiscent of the variability of red SR variables. An attempt to follow the period changes using the (O–C) method was unsuccessful because of the sparseness of the data. I therefore suggest that this star should be monitored systematically, preferably in UBV. It is well-placed for northern observers.