Eppur binaria non \`e esclusa: Pulsations in CZ Tuc appear to be tidally excited by its veiled companion

Long secondary periods (LSPs), seen in about one-third of pulsating red giants, remain unexplained despite decades of study, with binarity and non-radial oscillations as the leading competing hypotheses. If LSP variables are interacting eccentric binaries consisting of a red giant and a substellar object, tidally excited oscillations (TEOs) would be expected in a fraction of them. Detecting TEOs therefore offers an independent test of the binary scenario that had not previously been applied to LSP stars.

Using a combined, roughly 80-year light curve assembled from Digital Access to a Sky Century @ Harvard (DASCH), Hipparcos, and All Sky Automatic Survey (ASAS) photometry, the authors find that CZ Tucanae (CZ Tuc) exhibits a pulsation period of 55.4734 ± 0.0074 days along with its LSP of 443.62 ± 0.12 days. The ratio between these periods is 7.9970 ± 0.0024, consistent with a TEO.

Theoretical models show that the observed amplitude of the candidate TEO could be produced after adopting the estimated fundamental parameters of CZ Tuc. The findings identify CZ Tuc as the first candidate LSP star with a TEO, providing independent evidence for binarity among at least a subset of these enigmatic variables.