1996A&A...309..505Z

During the past twelve years we observed several spectra of the B[e] star R4 in the SMC with CASPEC at ESO, La Silla. We also obtained UBV(RI) and JHK photometry during various occasions at La Silla. Further photometry was available in the literature. Brightness variations typical for LBVs of about 0.7mag in V were found. The optical spectra are dominated by strong emission lines of hydrogen and of FeII and [FeII]. Also numerous absorption features of different ions, ranging from MgI to SiIV, were found. The radial velocity measurements clearly showed that R4 is a spectroscopic binary with an early B component showing absorption features of SiIII, SiIV, HeI, OII and NII, and an early A-type component with absorption lines mainly of TiII and CrII. The emission lines are associated with the B star which therefore is called B[e] component. Likewise the observed IR excess,a defining property of B[e] stars, is ascribed to this component. By extrapolating the radial velocity curves we determined an orbital period of about 21yrs, spectroscopic masses of M(sini)³≃13M_☉ for both, the B[e] and the A component, respectively, and their separation a=23a.u.. Using ATLAS8 models we estimated for the A star T_eff≃9500K from the strength of TiII, CrII, MgI and from an upper limit for FeI absorption lines, and M_bol≃-5.6. For the B[e] component we calculated the SiIII/SiIV ionization equilibrium and the spectral energy distribution using ATLAS8 model atmospheres. This yielded T_eff=27000K, logg=3.2, R=14R_☉ and M_bol=-7.7. From evolutionary tracks a ZAMS mass of 10M_☉ was estimated for the A star which is close to the spectroscopic mass. This leads to (sini)³≃1. The ZAMS mass of the B[e] component is about 20M_☉, i.e. nearly twice the present mass. Therefore this component has obviously lost a large fraction of its mass during previous evolutionary phases. With our observations we could for the first time determine directly the mass of a B[e] giant. The low present mass and the evolutionary timescales of both binary components show that the B[e] giant is in an advanced evolutionary stage, most likely in the post-red supergiant phase.