Long secondary periods (LSPs), observed in a third of pulsating red giant stars, are the only unexplained type of large-amplitude stellar variability known at this time. Here we show that this phenomenon is a manifestation of a substellar or stellar companion orbiting the red giant star. Our investigation is based on a sample of about 16,000 well-defined LSP variables detected in the long-term OGLE photometric database of the Milky Way and Magellanic Clouds, combined with the mid-infrared data extracted from the NEOWISE-R archive. From this collection, we selected about 700 objects with stable, large-amplitude, well-sampled infrared light curves and found that about half of them exhibit secondary eclipses, thus presenting an important piece of evidence that the physical mechanism responsible for LSPs is binarity. Namely, the LSP light changes are due to the presence of a dusty cloud orbiting the red giant together with the companion and obscuring the star once per orbit. The secondary eclipses, visible only in the infrared wavelength, occur when the cloud is hidden behind the giant. In this scenario, the low-mass companion is a former planet that has accreted a significant amount of mass from the envelope of its host star and grown into a brown dwarf.

在三分之一的脉动红巨星中观察到的长次级周期 (LSP) 是目前已知的唯一无法解释的大振幅恒星变异类型。在这里，我们表明这种现象是围绕红巨星运行的亚恒星或恒星伴星的表现。我们的调查基于在银河系和麦哲伦星云的长期 OGLE 光度数据库中检测到的约 16,000 个定义明确的 LSP 变量样本，并结合从 NEOWISE-R 档案中提取的中红外数据。从这个集合中，我们选择了大约 700 个具有稳定、大振幅、良好采样的红外光曲线的物体，发现其中大约一半表现出二次日食，从而提供了一个重要的证据，证明造成 LSP 的物理机制是二元性。即，LSP 光的变化是由于存在一团尘云围绕着红巨星和伴星一起运行，并在每个轨道上遮挡恒星一次。二次日食，仅在红外波长可见，发生在云隐藏在巨人身后时。在这种情况下，低质量伴星是一颗前行星，它从其宿主恒星的包层中吸积了大量质量并成长为棕矮星。