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Astron. Astrophys. 329, 579-582 (1998)

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2. Observations

2.1. Photometry

Differential observations were made in the four filters of the uvby system. HR 5718 (50 Boo) and HR 5676 ([FORMULA] Boo) were used as the comparison and check stars respectively. Observations were made during the periods February-March 1994 and March-July 1995 with the 0.75-m Four College Automated Photoelectric Telescope (FCAPT), then on Mt. Hopkins (AZ). The observing sequence starts with a measure of the dark count, and then in each filter the sky-ch-c-v-c-v-c-v-c-ch-sky, where sky is a reading of the sky background, ch that of the check star, c that of the comparison star and v that of the variable star. A total of 38 and 151 measurement were obtained, respectively, in each period. An additional 45 measures were obtained in the period 24-28 January 1994, with the 0.4-m telescope of the Círculo Astronómico del Mediterráneo (CAM) Observatory, located at Crevillente (Alicante, Spain), and equipped with the OPTEC SSP-5 photoelectric photometer. Readings were obtained sequentially through the four filters, following a sequence of the form c-v-sky-ch-v-sky-c-v-sky-ch... The mean differential values for the three observing periods, together with their standard deviations, are presented in Table 1. The complete set of individual measurements are given in Table 2.


Table 1. Differential mean values with their dispersion for the three observing runs

The mean values of the two FCAPT series are the same within the errors. Thus no long term variability with an amplitude greater than 0.01 mag. occurred during 1994-1995. No direct comparison with the CAM data is presented as the differential photometry is given in the instrumental system of each telescope.

To search for short term periodic variability we performed periodogram analyses separately for the data in each of the four uvby bandpasses, and for each observing run. Two periodogram techniques were used: a sine wave fitting and a phase dispersion minimization algorithm (PDM, Stellinwerf 1978). No significant peaks were apparent. Periodograms applied to the v-c and v-ch data presented the same aspect as the c-ch data. As a further check, for each set of data we drew composite light curves folded with the periods found by Guerrero et al. (1992). No apparent modulation is present in any of them. From the results of our analysis, periodic variability with an amplitude greater than 0.005 mag. can be excluded.

From the analyses of the light curves it was apparent that the photometric values for the day JD 2449779 were significantly fainter than average. A fading of about 0.02 magnitudes and a subsequent brightening is present in all four filters. In Fig. 1 we present the v-c and v-ch light curves for the y and u bands, together with the c-ch values for comparison. The mean differential values for the period have been subtracted to put the y and u values on the same scale. As the behavior in the b and v bands is similar, we have not shown them for simplicity. The total duration of the observation is about 0.15 days. Both at the beginning and the end of the light curve the observed values are below, although close, to the mean values for the period (value zero on the vertical axis). Hence we estimate the total duration of the fading event to be about 0.2 - 0.3 days. No other significant features are present in the light curves during the three observing periods.

[FIGURE] Fig. 1. Differential light curves of JD 2449779. Filled squares represent the y band values, and open squares the u band. Mean differential values have been substracted to put both bands on the same scale.

2.2. Spectroscopy

Spectra of [FORMULA] CrB were obtained during 1994 and 1995, in coincidence with the photometric observations with the 1.0-m Jacobus Kapteyn Telescope (JKT), located at the Observatorio del Roque de los Muchachos (La Palma, Spain), and equipped with the St Andrew's Richardson-Brealey Spectrograph (RBS) attached to its Cassegrain focus. Observations were made during four periods: 26 March 1994, 24-27 June 1994, 11-12 February 1995 and 5-7 August 1995. Different configurations were used, giving spectral resolutions between 1 and 2 Å. A total of 10 spectra were obtained, 4 centered on the H [FORMULA] line, 5 in the blue region (4000-5000 Å) including the H [FORMULA] and H [FORMULA] lines, and one in the infrared region (8300-9100 Å) including the higher Paschen series lines.

In these spectra no signs of activity are apparent. All the observed Balmer and Paschen series lines appear in pure absorption, without any evident emission feature. Two representative H [FORMULA] spectra are shown in Fig. 2.

[FIGURE] Fig. 2. H [FORMULA] spectra obtained on 26 June 1994 (top) and 11 February 1995 (bottom). Continua have been normalized to unity, and a value of 0.3 has been added to the top spectrum for display.

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997