Soszynski, I.Wood, Peter R2015-12-130004-637Xhttp://hdl.handle.net/1885/70631We analyze the distribution of semiregular variables and Mira stars in the period-luminosity plane. Our sample consists of 6169 oxygen-rich long-period variables in the Large Magellanic Cloud included in the OGLE-III Catalog of Variable Stars. There are many stars with periods that lie between the well-known sequences C and C′. Most of these stars are multi-periodic and the period ratios suggest that these stars oscillate in the same mode as the sequence C stars. Models suggest that this mode is the fundamental radial pulsation mode. The stars with primary periods between sequences C and C′ preferentially lie on an additional sequence (named F), and a large fraction of these stars also have long secondary periods (LSPs) that lie between sequences C and D. There are also a small number of stars with primary periods lying between sequences C and D. The origin of this long-period variability is unknown, as is the cause of sequence D variability. In addition, the origin of sequence F is unknown but we speculate that sequence F variability may be excited by the same phenomenon that causes the LSPs.Author/s retain copyrightKeywords: infrared: stars; stars: AGB and post-AGB; stars: late-type; stars: oscillations (including pulsations)201310.1088/0004-637X/763/2/1032016-02-24