2. The observations and data reduction
CCD images were obtained at six observing runs during 1991-1999 at Las Campanas Observatory (LCO) and ESO La Silla Observatory, Chile. Exposure times were between 250 and 300 s. Details of the observations are given in Table 1.
Table 1. Journal of observations. N is the number of science frames per night. HJD is the heliocentric julian day at the start of every sequence, referred to the zero point 244 0000. Comparison (C) and check (CH) stars are labeled accordingly to Fig. 1. The variances of the differences and light curve and the mean magnitude of RZ Leo are also given. Note the decrease of variability associated to the faint state of Jan-Feb 1998. Setup A refers to the 1.0 m LCO telescope and B to the Dutch 0.92 m ESO telescope.
All science images were corrected for bias and were flat-fielded using standard IRAF 1 routines. Instrumental magnitudes were calculated with the phot aperture photometry package, which is adequate due to the RZ Leo's uncrowded field. The optimum aperture radius defined by Howell (1992) was used. This radius matches the HWHM of the point spread function, minimizing the noise contribution due to sky pixels and readout noise.
In this paper we are interested in differential photometry. This technique, reviewed by Howell (1992), involves the determination of time series and , among instrumental magnitudes of variable (V), comparison (C) and check (CH) stars in the same CCD field. A finding chart of RZ Leo showing the check and comparison stars is shown in Fig. 1.
The photometric error of was derived from the standard deviation of the differences. In general, the intrinsic variance (not due to variability but to noise) associated to each differential light curve and are related by a scale factor depending on the relative brightness of the sources (Howell et al. 1988, Eq. 13). This factor is of order of unity if the three sources are of similar brightness or if the variable is of similar brightness to the check star and the comparison is brighter. These criteria are completely fulfilled in our observations.
Table 1 shows mean V magnitudes along with the comparison and check stars used every night. The star labeled in Fig. 1 (for which V = 14.201 is given by Misselt 1996), was used to shift the differences to an non-differential magnitude scale. On the other hand, UBV magnitudes taken at HJD 244 8333.5981, 244 8333.6044 and 244 8333.6131 were properly calibrated with photometric standard stars, yielding V = , = and = .
© European Southern Observatory (ESO) 1999
Online publication: November 23, 1999