Astron. Astrophys. 332, 541-560 (1998)

2. Observations

Our data set consists of 20 yearly light curves of AR Lac obtained from 1967 to 1992. Previous light curves were disregarded because the AR Lac magnitudes were derived from visual or photographic estimates or the comparison star was HD 209813 (BD 3572), lately recognized as the variable HK Lac by Blanco & Catalano (1968, 1970). In the present analysis we consider only the V band light curves because they are the most complete and homogeneous.

In addition to the standard correction for atmospheric extinction, all our magnitudes are referred to HD 208728 (BD 3741), the comparison star adopted by Rodonò et al. (1986). In Table 1 we present for each yearly light curve the mid epoch of the observations, the number of normal points (M), the aperture of the used telescope (), the photometric system (I instrumental, S standard UBV), the ephemeris adopted to compute the phase of the light curve and the literature reference.

Table 1. V-band light curves of AR Lac.

Due to the orbital period being very close to two days, it is difficult to obtain a good phase coverage of the light curve from a single observing site. Therefore points which appear to be near in phase could have been obtained after an interval of several weeks or a few months, during which the system might have undergone significant variations.

Indeed AR Lac is a very active RS CVn system and out-of-eclipse light variations up to 0.05 mag have been detected in the course of a single night. This intrinsic variability seems to have a somewhat larger amplitude during primary minima, when only the K0 IV component is visible. Srivastava (1986) studied in detail such variations in the V and B bands, attributing them to flares with timescales of 10-100 minutes. Due to the unfavourable orbital period and intrinsic light variations, a significant part of our light curves shows both phase gaps and large dispersion of the observations with an intrinsic standard deviation of mag. In order to avoid a loss of information by averaging such variations, whenever possible the available observations were not averaged into normal points for the subsequent analysis. Moreover, all points in the AR Lac light curves were given the same unity weight, whereas in the previous analysis of the RS CVn light curves we used normal points with different weights (see Rodonò et al. 1995).

The brightest magnitude of AR Lac from our light curve sequence turned out to be at orbital phase 0.7395 in the 1987 light curve. This value is assumed as the magnitude of the unspotted system at phase 0.7395, thus establishing the magnitude scale calibration for the subsequent light curve modelling.

The V-band data from Catania Observatory used in the present paper will be described in detail in a forthcoming paper dealing with long-term UBV photometry of AR Lac.

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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