Long secondary periods of 2,877 LMC red giants lose phase between OGLE 1997-2009 and Gaia DR3 2014-2017, while short-period ellipsoidal binaries keep phase

Question: does the long secondary period (LSP) of LMC red giants keep a constant phase over two decades, as a light change locked to a binary orbit must do? Approach: for 2,877 OGLE-III LSP stars we fitted the OGLE-II/III I-band light curves (1997-2009), predicted the LSP phase at the middle of the Gaia DR3 epoch photometry (2014-2017), and measured the Gaia G phase at the same period. For each star we injected a strictly periodic signal into the same data to get the measurement noise. Red-giant ellipsoidal binaries from the same catalogue and the pulsation periods of the LSP stars are controls. Results: the median absolute LSP phase offset is 0.121 cycles against a median null SD of 0.053 cycles; 52% of the stars exceed the strictly periodic null at p <= 0.05. A random-walk phase model gives an effective diffusion D = 0.0045 cycles^2 per cycle (95% CI 0.0043-0.0048), which light-curve injections convert to an intrinsic D of about 0.003. Ellipsoidal binaries with periods below 200 d keep phase over a median of 19 cycles (D = 3e-5, upper limit 7e-5), and all ellipsoidal stars at Gaia S/N >= 10 give D <= 1e-4. A same-band test inside the OGLE data gives LSP D = 0.0043 for 4,953 stars and D = 0 for 444 ellipsoidal stars. The early and later drifts are not correlated, so the data agree with a stochastic phase wander, not a constant period change. Pulsation periods keep no phase memory. Conclusion: in this scope the photometric LSP is not a strict clock; a binary model needs obscuring material that moves in orbital longitude. Limitations: different bands (I against G), sparse Gaia sampling (median 36 epochs), a null that is somewhat optimistic for faint signals, LMC only, and no radial-velocity test.

long secondary period · red giants · Large Magellanic Cloud · OGLE · Gaia DR3 · phase coherence · binary hypothesis · ellipsoidal variables · period stability · variable stars