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Astron. Astrophys. 329, L61-L63 (1998)

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1. Introduction

AG Dra is a well-studied bright symbiotic binary. The SIMBAD database lists 173 bibliographic references between 1983 and 1997. The orbital period of about 550 days is well established from radial-velocity data (e.g. Garcia & Kenyon 1988). The orbital inclination was determined by Schmid & Schild (1997) from variations in the polarization of optical Raman lines. On the basis of the high radial velocity (-140 km/s), the low metallicity ([Fe/H] [FORMULA] -1.5) and the faintness of CNO lines in IUE spectra, Schmid & Nussbaumer (1993) assigned the system to the old halo population. The cool component is a K giant which is well within its Roche limits (Garcia & Kenyon 1988). It is enriched in heavy s-process elements (Schmid & Schild 1997). The white dwarf has a surface temperature of about [FORMULA]  K (e.g. Greiner et al. 1996).

The optical variability of AG Dra used to be characterized by small irregular variations and, in addition, bright outburst phases. The burst phases repeat in intervals of about 15 years (1936, 1951, 1966, 1980-82, 1994-97), with only one small interloper (1985). In the visible spectral range there is no photometric signature of the orbital period (which was determined by radial-velocity measurements for the cool component), but it clearly shows up in the near UV (Meinunger 1979, Skopal & Chochol 1994, Skopal 1994) in the form of minima at the lower conjunction of the cool component (Skopal 1994 and references therein).

The photometric periodicity to be reported in the following was suspected already in 1994 (Bastian 1995) at the start of the present outburst phase, in a call for observations to amateurs. The present paper presents the confirmation of that suspicion by the behaviour of the system in the two years since then, and by the use of Hipparcos photometry.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997