Variability and spectral classification of LMC giants: Results from DENIS and EROS

We present the first cross-identifications of sources in the near-infrared DENIS survey and in the micro-lensing EROS survey in a field of about 0.5 s quare degrees around the optical center (OC) of the Large Magellanic Cloud. We analyze the photometric history of these stars in the EROS data base an d obtain light-curves for about 800 variables. Most of the stars are long p eriod variables (Miras and Semi{Regulars); a few Cepheids are also present. We also present new spectroscopic data on 126 asymptotic giant branch star s in the OC field, 30 previously known and 96 newly discovered by the DENIS survey. The visible spectra are used to assign a carbon- (C-) or oxygen{ri ch (O-rich) nature to the observed stars on the basis of the presence of mo lecular bands of TiO, VO, CN, C-2. For the remaining of the stars we used t he (J - K-s) color to determine whether they are O-rich or C-rich. Plotting Log(period) versus K-s we find three very distinct period-luminosity relat ions, mainly populated by Semi-Regular of type a (SRa), b (SRb) and Mira va riables. Carbon{rich stars occupy mostly the upper part of these relations. We find that 65% of the asymptotic giant branch population are long period variables (LPVs).