PULSATION MODES OF MIRA STARS AND QUESTIONING OF LINEAR MODELING - INDICATIONS FROM HIPPARCOS AND THE LMC

Thorough discussion of the previous theoretical works on the pulsation of Long Period Variables leads us to the conclusion that the mode per iods predicted by linear models must significantly differ from the rea lity, and that, if one nevertheless relies upon such a modelling, it i s at least necessary to change the mixing length. The hypothesis that the so-derived mode periods be reasonably reliable is supported by con frontation between a model grid based on these grounds and the luminos ities of LPVs in the Large Magellanic Cloud and in LMC clusters, as we ll as the luminosities and effective temperatures of Miras in the sola r neighbourhood. A wide majority of the Miras appear probably pulsatin g on the first overtone, and the sample Semi-Regulars on the second. H owever, a significant proportion of Miras seem to be fundamental pulsa tors. Individual masses are derived. A few stars are probably undergoi ng hot bottom burning, while two seem to have a peculiar dust envelope .