Improved model of the triple system V746 Cassiopeiae that has a bipolar magnetic field associated with the tertiary

V746 Cas is known to be a triple system composed of a close binary with an alternatively reported period of either 25d:4 or 27d:8 and a distant third component in a 170 yr (62 000 d) orbit. The object was also reported to exhibit multiperiodic light variations with periods from 0d: 83 to 2d: 50, on the basis of which it was classified as a slowly pulsating B star. Interest in further investigation of this system was raised by the recent detection of a variable magnetic field. Analysing spectra from four instruments, earlier published radial velocities, and several sets of photometric observations, we arrived at the following conclusions: (1) The optical spectrum is dominated by the lines of the B-type primary (Teff 1 16 500(100)K), contributing 70% of the light in the optical region, and a slightly cooler B tertiary (Teff 3 13 620(150)K). The lines of the low-mass secondary are below our detection threshold; we estimate that it could be a normal A or F star. (2) We resolved the ambiguity in the value of the inner binary period and arrived at a linear ephemeris of Tsuper: conj: = HJD 2 443 838:78(81) + 25d:41569(42) × E. (3) The intensity of the magnetic field undergoes a sinusoidal variation in phase with one of the known photometric periods, namely 2d:503867(19), which we identify with the rotational period of the tertiary. (4) The second dominant photometric 1d: 0649524(40) period is tentatively identified with the rotational period of the broad-lined B-type primary, but this interpretation is much less certain and needs further verification. (5) If our interpretation of photometric periods is confirmed, the classification of the object as a slowly pulsating B star should be revised. (6) Applying an N-body model to different types of available observational data, we can constrain the orbital inclination of the inner orbit to 60° < i1 < 85° even in the absence of binary eclipses, and we estimate the probable properties of the triple system and its components.250015 GA15-02112S Univerzita Karlova v Praze: GA17-00871SAcknowledgements. We profited from the use of 13 echelle spectra from the Bernard Lyot telescope, made publicly available via the Polar Base web service and Geneva 7-C photometry made available via HELAS service, to which we were kindly directed by C. Aerts. P. Mathias kindly provided us with old archival DAT tapes with the original Aurelie spectra of ten B stars and some advice about their content and structure. The spectra were reconstructed from the tapes with the help of R. Vesely. H. Bozic, K. Hon^ková, and D. Vršnjak kindly obtained some of the calibrated UBV observations of V746 Cas and its comparison stars for us. We acknowledge the use of the public versions of programs FOTEL and KOREL, written by P. Hadrava and PHOEBE 1.0 written by A. Prša. The research of P.H., M.B., P.M., and J.N. was supported by the grant P209/10/0715 of the Czech Science Foundation. J.N. and P.H. were also supported by the grants GA15-02112S of the Czech Science Foundation and No. 250015 of the Grant Agency of the Charles University in Prague. Research of D.K. was supported by a grant GA17-00871S of the Czech Science Foundation. Our thanks are due to M. Wolf, who obtained one Ond^ejov spectrum used here and who provided a few useful comments to this paper. A persistent but constructive criticism by an anonymous referee helped us to re-think the whole study, present our arguments and analyses more clearly and convincingly, and to improve the layout of the text and figures as well. The use of the NASA/ADS bibliographical service and SIMBAD electronic database are gratefully acknowledged. -