Red variables in the OGLE-II data base – I. Pulsations and period–luminosity relations below the tip of the red giant branch of the Large Magellanic Cloud

We present period–luminosity relations for more than 23 000 red giants in the Large Magellanic Cloud observed by the OGLE-II microlensing project. The OGLE period values were combined with the 2MASS single-epoch JHKS photometric data. For the brighter stars we find agreement with previous results (four different sequences corresponding to different modes of pulsation in asymptotic giant branch stars). We also discovered two distinct and well-separated sequences below the tip of the red giant branch. They consist of almost 10 000 short-period (15 < P < 50 d), low-amplitude (AI < 0.04) mag red variable stars, for which we propose that a significant fraction is likely to be on the red giant branch, showing radial pulsations in the second and third overtone modes. The excitation mechanism could be either Mira-like pulsation or solar-like oscillations driven by convection.