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Astron. Astrophys. 323, 113-120 (1997)

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The symbiotic star YY Herculis

I. Photometric history over 1890-1996 *

U. Munari 1, M. Rejkuba 2, 3, M. Hazen 4, J. Mattei 5, E. Schweitzer 6, R. Luthardt 7, S. Shugarov 8, B.F. Yudin 8, A.A. Popova 8, P.V. Chugainov ** 9, G. Sostero 10 and A. Lepardo 10

1 Osservatorio Astronomico di Padova, Sede di Asiago, I-36012 Asiago (VI), Italy (munari@astras.pd.astro.it)
2 Osservatorio Astrofisico del Dipartimento di Astronomia, Università di Padova, I-36012 Asiago (VI), Italy (rejkuba@astras.pd.astro.it)
3 Faculty of physics, University of Zagreb, Bijenika 32, 10000 Zagreb, Croatia
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA (mhazen@cfa.harvard.edu)
5 The American Association of Variable Star Observers, 25 Birch Street, Cambridge, MA 02138, USA (jmattei@aavso.org)
6 A.F.O.E.V., Observatoire de Strasbourg 11, rue de l'Université, F-67000 Strasbourg, France (afoev@astro.u-strasbg.fr)
7 Sonneberg Observatory, Sternwartestr. 32, D-96515 Sonneberg, Germany (rlu@stw.tu-ilmenau.de)
8 Sternberg Astronomical Institute, University of Moscow, Universitetsky prospect 13, 119899 Moscow, Russia (yudin@sai.msu.su, shugarov@sai.msu.su, popova@sai.msu.su)
9 Crimean Astrophysical Observatory, Nauchny 334413, Crimea, Ukraine
10 Associazione Friulana di Astronomia e Meteorologia, C.P. 179, I-33100 Udine, Italy (ossastr@ten.it)

Received 6 November 1996 / Accepted 19 December 1996

Abstract

The lightcurve of the symbiotic star YY Her extending from 1890 to 1996 is presented and discussed.

YY Her underwent four outbursts in the last century, in 1914-19, 1930-33, 1981-82 and 1993-96, with additional bright phases in 1890, 1903, 1942, 1954, 1965 and 1974. The absolute magnitude reached at peak brightness is adequate for mild thermonuclear flashes with no mass loss induced by super-Eddington luminosities.

An orbital period of 590 days is inferred from the quiescence [FORMULA] =0.3 mag modulation ascribed to a heating effect. Four enigmatic deep minima in 1952, 1965, 1971 and 1992 appeared at the right phase for an eclipse to occur, but such an explanation encounters severe difficulties. At least two of these minima occurred right at the onset of an active phase or outburst and this suggests an alternative interpretation to the eclipse hypothesis.

YY Her appears to belong to the Bulge/thick-Disk population of the Galaxy, with [FORMULA] kpc, [FORMULA] pc and [FORMULA] =0.41

Key words: symbiotic binaries – individual stars: YY Her

* Tables 5 and 6 are only available in electronic form at the CDS via anonymous ftp 130.79.128.5
** Late (1992)

Send offprint requests to: U.Munari

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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