Variable C : an S Doradus-type variable in M 33.

Abstract

Variable C is one of the Hubble-Sandage variables in M 33. Our photometric and spectroscopic observations obtained during one of its rare maxima show that Variable C's variations are typical of luminous blue variables especially those of the S Doradus type. Its spectrum at maximum light suggests a temperature of 7500 K for its expanding optically thick pseudo-photosphere which may be the coolest temperature yet measured for a luminous blue variable at maximum. We find a mass loss rate of 4 10^-5^ M_sun_yr^-1^ from the infrared excess and from the Hα emission which is typical of these variables at maximum. Its bolometric luminosity is -9.8 mag and at maximum M_v_ is also -9.8 mag.