FIRST RESULTS O P A N INTERNATIONAL MULTISITE MULTITECHNIQUE CAMPAIGN ON omicron AND J.P. SAREYAN 1 , J. CHAUVILLE 2 , D. BRIOT 3 , S.J. ADELMAN, M. ALVAREZ, I. BALEGA, Y. BALEGA, D. BARDIN, I. BELKIN, D. BONNEAU, M. BOSSI, V. DESNOUX, M. ESPOTO, R. FRIED, S. GONZALEZ-BEDOLLA, X.Z. GUO, Y.L. GUO, J.X. HAO, L. HUANG, A. KLOTZ, . LEISTER, F. MORAND, D. MOURARD, F. VAKILI and F. VALERA, F.Y. ZHAO 1 Observatoire de la Cote d Azur, 06304 N* ce Cedex 04, Prance 2 Observatoire de Meudon, 92195 Meudon Cedex, Prance 8 Observatoire de Paris, Av.de l Observatoire, 75014 Poris, Prance Omicron Andromedae is a multiple system of at least four stars : a <->Be star (component A), a spectroscopic binary (components B1-B2) and a close companion (component a). According to several studies (see Hill et al. 1988, 1989) : - the distance between A and the B1-B2 system decreased from 0.39" in 1975 to 0.25" in 1987 (McAlister and Hartkopf 1988) - the few previous speckle measurements of component a have shown the possibility of a 3.7 years orbit around A, according to the 1975 to 1984 observations (mean distance 0.05"). The calculations with this 3.7 yr orbit lead to the prediction of a maximum distance of 0.77" at 1992.738, i.e. at the end of September 1992, with a North-South orientation. But a strong contradiction appears : given the spectroscopic distance (188 parsecs), this orbit would lead to a mass of 180 for the -a system; or if we adopt a mass of 7 M for A, we deduce an orbital period of 14 years for -a, which cannot fit the speckle positions. From a photometric point of view, the Be (with an eventual shell phase) <-> switching occurs every 4 to 9 years (with a 8.5 years average), and can be described as follows : - when the star is at its light minimum (Be phase, lasting from 2 to 7 years), it displays a double-wave light curve of 1.6 day period, with a 0.1 mag amplitude in the visible. - it has been recently shown (Sareyan et al., 1992) that this 1.6 day period still exists when the star is in its "normal" phase (the star being then bluer), but with a 0.01 mag amplitude, i.e. very difficult to detect with such a long period. We decided of a campaign on this star with several techniques, in order to solve the contradictions mentioned above, and also to get a better under- standing of this rather complex system, i.e. investigating the relationship 282 L. A. Balona et al (eds.), Pulsation, Rotation and Mass Loss in Early-Type Stars, 282-283. 19941AU. Printed in the Netherlands. available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S007418090021509X Downloaded from https://www.cambridge.org/core. IP address: 3.236.55.199, on 09 Jun 2020 at 03:06:11, subject to the Cambridge Core terms of use,