Long secondary periods and binarity in red giant stars

Observational arguments supporting the binary explanation of the long secondary period ( LSP) phenomenon in red giants are presented. Photometry of about 1200 semiregular variables with LSPs in the Large Magellanic Cloud is analyzed using the MACHO and OGLE photometry. For about 5% of these objects, additional ellipsoidal-like or eclipsing-like modulation with the same periods as the LSP is detectable. These double-humped variations are usually shifted in phase compared to LSP light curves. I discuss the model of a binary system with a red giant as the primary component and a low-mass object as the secondary. The mass lost by the red giant through the wind follows a spiral pattern in the orbit around the primary star and obscures it, causing LSP variations.