R4 IN THE SMALL MAGELLANIC CLOUD - A SPECTROSCOPIC BINARY WITH A B[E]LBV-TYPE COMPONENT/

During the past twelve years we observed several spectra of the B[e] s tar R4 in the SMC with CASPEC at ESO, La Silla. We also obtained UBV(R I) and JHK photometry during various occasions at La Silla. Further ph otometry was available in the literature. Brightness variations typica l for LBVs of about 0.7(m) in V were found. The optical spectra are do minated by strong emission lines of hydrogen and of Fe II and [Fe II]. Also numerous absorption features of different ions, ranging from Mg I to Si IV, were found. The radial velocity measurements clearly showe d that R4 is a spectroscopic binary with an early B component showing absorption features of Si III, Si IV, He I, O II and N II, and an earl y A-type component with absorption lines mainly of Ti II and Cr II. Th e emission lines are associated with the B star which therefore is cal led B[e] component. Likewise the observed IR excess, a defining proper ty of B[e] stars, is ascribed to this component. By extrapolating the radial velocity curves we determined an orbital period of about 21 yrs , spectroscopic masses of M sin(3) i approximate to 13 M. for both, th e B [e] and the A component, respectively, and their separation alpha = 23 a.u.. Using ATLAS8 models we estimated for the A star T-eff appro ximate to 9500 K from the strength of Ti II, Cr II, Mg I and from an u pper limit for Fe I absorption lines, and M(bol) approximate to -5.6. For the B[e] component we calculated the Si III/Si IV ionization equil ibrium and the spectral energy distribution using ATLAS 8 model atmosp heres. This yielded T-eff = 27 000 K, log g = 3.2, R = 14 R. and M(bol ) = -7.7. From evolutionary tracks a ZAMS mass of 10 M. was estimated for the A star which is close to the spectroscopic mass. This leads to sin(3) i approximate to 1. The ZAMS mass of the B[e] component is abo ut 20 M., i.e. nearly twice the present mass. Therefore this component has obviously lost a large fraction of its mass during previous evolu tionary phases. With our observations we could for the first time dete rmine 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] g iant is in an advanced evolutionary stage, most likely in the post-red supergiant phase.