2 Observations and period analysis

In order to enlarge the number of times of minimum light, new observations for all three systems were carried out. Our new photoelectric photometry was performed at three observatories with the aim of securing at least two well-covered minima for each variable:

$\bullet$ South Africa Astronomical Observatory (SAAO), Sutherland, South Africa: 50 cm Cassegrain telescope with Johnsons UBV filters and classical photometer;

$\bullet$ Ondrejov Observatory, Czech Republic: 65 cm reflecting telescope with CCD camera SBIG ST-8 and Cousins R filter;

$\bullet$ private observatory of K.H. at Lelekovice, Czech Republic: 35 cm Newtonian telescope with CCD camera SBIG ST-6V and R filter.

Photoelectric observations at SAAO were obtained with the modular photometer utilizing a Hamamatsu GaAs R943-02 photomultiplier during two weeks in August 1999. Each observation of an eclipsing binary was accompanied by observation of a local comparison star. The photoelectric measurements were done in the UBV filters of the Johnson's photometric system with a 10 s integration time. All observations were reduced to the Cousins E-region standard system (Menzies et al. 1989).

The CCD measurements in Ondrejov were done using the standard Cousins R filter. Flat fields for the reduction of the CCD frames were routinely obtained from exposures of regions of the sky taken at dusk or dawn. Several comparison stars were chosen on the same frame as the variables. During the observations, no variations in the brightness of these stars exceeding the possible error of measurements (typical $\sigma \simeq 0.005$ mag) were detected. No correction was performed for differential extinction, due to the proximity of the comparison stars to the variable and the resulting negligible differences in the air mass.

The new times of primary and secondary minimum and their errors were determined using the least squares fit to the data by the bisecting chord method. These times of minimum are presented in Table 1.

 

 

Table 1: New precise times of minimum light

System	JD Hel.-	Error	N	Method	Observatory
	2400000	[day]		Filter
DK Cyg	47963.662*	0.001	15	H	Hipparcos
	47963.896*	0.001	16	H	Hipparcos
	48297.616*	0.001	14	H	Hipparcos
	48302.793*	0.001	10	H	Hipparcos
	51379.4817	0.0001	54	CCD, -	Jindr. Hradec
	51749.4447	0.0001	76	CCD, R	Lelekovice
V401 Cyg	48254.0593	0.0005	10	H	Hipparcos
	51333.46434	0.0001	88	CCD, R	Ondrejov
	51391.44435	0.0001	54	CCD, R	Ondrejov
	51394.360	0.0005	15	CCD, R	Ondrejov
	51680.48045	0.00007	70	CCD, R	Ondrejov
	51738.4647	0.0001	75	CCD, R	Ondrejov
AD Phe	48126.759*	0.001	11	H	Hipparcos
	48480.4675	0.001	6	H	Hipparcos
	48480.658*	0.001	18	H	Hipparcos
	48540.496*	0.001	12	H	Hipparcos
	48540.686*	0.001	16	H	Hipparcos
	51412.5292	0.0005	13	pe, BV	SAAO
	51417.46805	0.0001	13	pe, BV	SAAO
	51418.41447	0.0001	37	pe, BV	SAAO
Y Sex	48068.526*	0.001	10	H	Hipparcos
	51655.4420	0.0005	44	CCD, R	Lelekovice
	51663.4184	0.0001	66	CCD, R	Lelekovice

* Normal minimum.

From the Hipparcos photometry (Perryman 1997), we were able to determine several additional moments of minimum light. They are given also in Table 1. In this table N stands for the number of measurements used for the determination of minimum time.

Period changes in all systems were studied by means of an O-C diagram analysis. We have collected all reliable times of minimum light gathered from the literature, from current databases of AAVSO, BAV and BBSAG observers as well as from the Besançon Double and Multiple Star Database. We employed the following data reduction procedure. All photoelectric times of minimum were used with a weight of 10 in our computation. The current less precise measurements were weighted with a factor of 5, while the earlier visual and photographic times of minimum were given a weight of 1 due to the large scatter in these data.