| Abstract | From recent detailed discussions of S Doradus variables evidence has been provided that these luminous blue variables occupy at quiescence an inclined instability strip in the HRD in the luminosity-temperature range -9 <= M_bol_ <= -11 and 14000 <= T_eff_ <= 35000 K. The most luminous S Dor variables are the hottest ones. During outbursts dense envelopes are expelled and at maximum all S Dor variables are characterized by virtually the same spectral appearance of the slowly expanding (v_exp_ = 100 to 200 km s^-1^) dense envelopes of equivalent spectral type middle A to early F. S Dor variables occupy at maximum a vertical strip in the HRD at T~8000 K, called opaque wind limit. Since normal S Dor type eruptions occur under the condition M_bol_~const, the observed photometric variations in the visual and blue spectral range are virtually variations of B.C. As the instability strip is inclined the temperature difference between S Dor variables at quiescence and at maximum stage increases with L. Hence, the photometric amplitudes increase with L, indicating the existence of an amplitude-luminosity relation. A numerical expression for this relation is given. The absolute calibration is derived from the LMC-S Dor variables with well known distances. The relation is used to derived distances to other galaxies. |
