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Paper: Secular Pulsation Amplitude Variations in the Subdwarf-B Star Component of the V4640 Sgr Binary
Volume: 479, Progress in Physics of the Sun and Stars
Page: 273
Authors: Lynas-Gray, A. E.
Abstract: V4640 Sgr (EC 20117–4014) is understood to be a long-period binary consisting of a V361 Hya type pulsating subdwarf-B star and a F5 V star. Observations obtained before 2010 show the amplitude of the dominant power spectrum peak (7.28 mHz) to be constant when data from all nights in each season are combined and analysed as a single time-series. Because of the need to constrain mass-transfer scenarios in binaries like V4640 Sgr when the subdwarf-B star progenitor was at or near the Helium Flash, this object was selected for further photometric monitoring with the intention of using a changing light travel-time to improve the orbit determination. An analysis of 2011 August, and existing 2001 July, time series show the 7.28 mHz frequency to have secular nightly amplitude changes. Comparing with similar pulsation amplitude changes in V541 Hya and KIC 010139564 suggested these are a consequence of unresolved rotation splitting, with two or more split components beating against each other. Modelling for confirmation, and assessment of any associated frequency changes, would be needed before any refinement of the light travel-time orbit determination.
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