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Astron. Astrophys. 364, 543-551 (2000)

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1. Introduction

The eclipsing binary AB And is a well observed variable star, which belongs to the W subgroup of the W UMa-type systems. Its variability was discovered by Guthnick & Prager (1927). After its discovery, AB And was observed quite often. Thus, some asymmetries in its light curves as well as some seasonal variations have been reported (e.g. Rovithis-Livaniou & Rovithis 1981; Bell et al. 1984; Demircan et al. 1994). In the latter, one can also find a list of the existing previous observations of the system.

The spectrum of AB And was classified as G5 by Struve et al. (1950) and as G5n by Hill et al. (1975), while according to Landolt (1969), the eclipsing star was probably an unreddened K2 main sequence star. Finally, Hrivnak's (1988) new spectroscopic observations confirmed the early classification yielding G5, but with a different value of the mass ratio.

The present study deals with the problem of estimating AB And's orbital and physical parameters. It is based on the interpretation of its photometric observations, using the data from 1968 till 1995. So, the light curves by Landolt (1969), Rigterink (1973), Bell et al. (1984) and those obtained at the Kryonerion Astronomical Station during 1978, 1990 and 1995 are analysed and discussed.

Landolt's observations were made at the Kitt Peak National Observatory (No. 4 16-inch reflecting telescope) during September and October 1968. Standard B and V filters were used with a 1P21 photomultiplier. [FORMULA] was used as a comparison star. The average external probable errors of the photometry were estimated to be [FORMULA] in V and [FORMULA] in [FORMULA]. The asymmetry in the light curve is evident. During August and October 1970 at Kitt Peak Rigterink acquired a large set of B and V observations using the same equipment and the same comparison star. These observations give quite symmetric light curves of AB And. Bell's observations were made during October 1982, with the 0.5-m Leslie Rose telescope at the University Observatory, St Andrews, which is an f /11 Ritchey-Chretien system. For photometric observations a standard two-channel Peoples Photometer is employed, with S-20 response EMI9558 photomultipliers in thermoelectrically-controlled cold boxes. The comparison star was [FORMULA]. The light curve show significant asymmetry, with the second quadrature [FORMULA] mag fainter than the first one.

Photoelectric observations of AB And during 1978, 1990 and 1995 were made using the two-beam, multi-mode, nebular-stellar photometer of the National Observatory of Athens attached to the [FORMULA] Cassegrain reflector at Krionerion Astronomical Station. The B and V filters used are in close agreement with the standard UBV system. The star [FORMULA] was used for comparison.

The analysis of the O-C curve of AB And shows short and long-time period variations (Kalimeris et al. 1994). Because of that the orbital phases of the light curves are calculated by using different ephemeris formulae. These are:

Landolt (1969)

[FORMULA]

Rigterink (1973)

[FORMULA]

Kholopov (1985)

[FORMULA]

With the first formula the orbital phases of Landolt's 1968 observations were calculated. Bell's 1982 light curves were assembled on the basis of the times of primary minima given in Bell et al. (1984), and Landolt's orbital period. The second formula is used for Rigterink's 1970 observations and the third for the observations made at Krionerion Astronomical Station during 1978. For the observations made during 1990 and 1995 we used the third formula, but with a modified period, i.e.:

1990

[FORMULA]

1995

[FORMULA]

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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