4. Primary comparison stars (C₁)

The C₁ of each SET is given in Table 3 (Col. 4). The two most frequently used C₁ are SAO 50313 and SAO 50205. The third one has been used only once (SET=2: SAO 50260).

1. Primary comparison star SAO 50313

The spectral-type estimates of SAO 50313 (HD 199956, $\Delta_{\mathrm{sep}}\!=\!1\hbox{$.\!\!^\circ$ }0$) range between G8III and K1III (Häggkvist & Oja 1970; Straizys et al. 1989). The short-term constant brightness  was thoroughly verified by Gonzalez B. et al. (1980: their Fig. 2), who used it as C₁in confirming that the possible pulsations of classical Am-stars cause no detectable photometric variability. We detected no short-term variability, nor periodicity between $0\hbox{$.\!\!^{\rm d}$ }4$ and 50^d, in our new data of SAO 50313. Our measurements indicate constant long-term BV brightness (Fig. 1 and Table 2). The weighted long-term U mean is inaccurate ( $\sim \! 0\hbox{$.\!\!^{\rm m}$ }046$), but comparable to the external accuracy ( $\sim \! 0\hbox{$.\!\!^{\rm m}$ }03$). The $\Delta U \approx 4\hbox{$.\!\!^{\rm m}$ }5$ difference between 57 Cyg and SAO 50313 probably induces this scatter in the APT photometry. It may also indicate low accuracy for the transformations into the standard Johnson system. The BV remain constant during some apparently large U changes (e.g. SET=33 and 36). But UBV changes should correlate, were the U changes induced by starspots (e.g. Paper  II: Fig. 3). Only one R measurement of SAO 50313 has been made, and none in I. In conclusion, the long-term constant brightness was established in BV, but not in U. The onset of photometric variability in late-type stars as a function of the Rossby number has been studied, e.g., by Hall (1991). Thus the determination of $v \sin{i}$ of this late-type C₁ might give a rough estimate of whether brightness variations due to starspots could even be expected.

2. Primary comparison star SAO 50205

SAO 50205 (HD 199206, $\Delta_{\mathrm{sep}}\!=\!0\hbox{$.\!\!^\circ$ }7$) is an early-type B8II close visual binary also known as ADS 14411AB (Fehrenbach et al. 1961). Estimates of the magnitude difference and the angular separation between the A and B components range from ${\Delta}V \! = \! 1\hbox{$.\!\!^{\rm m}$ }70$ to $1\hbox{$.\!\!^{\rm m}$ }82$, and from $2\hbox{$.\!\!^{\prime\prime}$ }53$ to $2\hbox{$.\!\!^{\prime\prime}$ }80$(Rakos et al. 1982; Turon et al. 1992). This angular separation should not cause observational errors in photometry. For example, the diaphragm diameters of the APT Phoenix 10 inch telescope at the Mount Hopkins Observatory and the AZT-14 telescope at the Mount Maidanak Observatory are 60'' and 22'', respectively. Our new SAO 50205 data confirm constant long-term UBV brightness with a high precision (Fig. 2). The three R measurements are in overall agreement, but the single I measurement is inaccurate. SAO 50205 has shown no short-term photometric variability (Rakos et al. 1982). No significant periodicity was detected between $0\hbox{$.\!\!^{\rm d}$ }4$ and 50^d, nor irregular short-term variability in our extensive APT data. Hence the early-type SAO 50205 is a reliable C₁ for V 1794 Cyg.

3. Primary comparison star SAO 50260

SAO 50260 (HD 199547, K0-2III, $\Delta_{\mathrm{sep}}\!=\!0\hbox{$.\!\!^\circ$ }6$) is a long-period ( $P_{\mathrm{orb}}=2871^{\mathrm{d}} \pm 14^{\mathrm{d}}$) late-type spectroscopic binary (Fehrenbach et al. 1961; Häggkvist & Oja 1970; Griffin 1984). Short-term constant brightness has not been verified. The long-term UBV magnitudes suggest no variability, but the RI magnitudes are unknown (Table 2). SAO 50260 was used as C₁ only once (SET=2). Because the two most frequently used C₁ seem reliable (SAO 50313 and SAO 50205), using SAO 50260 as C₁ of V 1794 Cyg is unnecessary.

   

Table 2: The UBVRI magnitudes of all C₁ and their weighted long-term means. YEAR is the SET mean epoch

SAO 50313
YEAR	U	B	V	R	I	Reference
	$8.700 \pm 0.029$	$7.730 \pm 0.018$	$6.660 \pm 0.012$			Landolt 1975
			$6.640 \pm 0.010$			Straizys et al. 1989
		$7.730 \pm 0.034$	$6.660 \pm 0.031$			Turon et al. 1992
1982.84	$8.642 \pm 0.018$	$7.697 \pm 0.014$	$6.624 \pm 0.016$			SET= 7, Paper  II
1983.34	$8.695 \pm 0.042$	$7.744 \pm 0.024$	$6.670 \pm 0.021$			SET= 8, Paper  II
1983.64	$8.693 \pm 0.009$	$7.707 \pm 0.017$	$6.643 \pm 0.014$			SET= 9, Paper  II
1984.33	$8.674 \pm 0.074$	$7.710 \pm 0.041$	$6.643 \pm 0.032$			SET=11, Paper  II
1984.70	$8.712 \pm 0.032$	$7.713 \pm 0.009$	$6.634 \pm 0.005$			SET=13, Paper  II
1985.39	$8.687 \pm 0.046$	$7.724 \pm 0.023$	$6.643 \pm 0.021$			SET=14, Paper  II
1988.74	$8.561 \pm 0.044$	$7.748 \pm 0.022$	$6.667 \pm 0.031$			SET=33, Paper  II
1988.80	$8.621 \pm 0.039$	$7.724 \pm 0.013$	$6.633 \pm 0.020$			SET=34, Paper  II
1988.86	$8.605 \pm 0.041$	$7.731 \pm 0.016$	$6.642 \pm 0.020$			SET=35, Paper  II
1988.93	$8.572 \pm 0.041$	$7.724 \pm 0.018$	$6.639 \pm 0.019$			SET=36, Paper  II
1989.36	$8.637 \pm 0.037$	$7.714 \pm 0.015$	$6.626 \pm 0.020$			SET=37, Paper  II
1989.45	$8.629 \pm 0.048$	$7.715 \pm 0.014$	$6.624 \pm 0.021$			SET=39, Paper  II
1989.72	$8.578 \pm 0.035$	$7.702 \pm 0.017$	$6.632 \pm 0.020$			SET=43, This paper
1989.79	$8.570 \pm 0.034$	$7.695 \pm 0.014$	$6.625 \pm 0.023$			SET=44, This paper
1989.86	$8.596 \pm 0.037$	$7.695 \pm 0.014$	$6.622 \pm 0.018$			SET=46, This paper
1989.91	$8.575 \pm 0.026$	$7.681 \pm 0.017$	$6.621 \pm 0.015$	$5.816 \pm 0.010$		SET=47, This paper
1989.94	$8.642 \pm 0.036$	$7.712 \pm 0.014$	$6.637 \pm 0.020$			SET=48, This paper
1990.38	$8.596 \pm 0.036$	$7.703 \pm 0.012$	$6.631 \pm 0.020$			SET=49, This paper
1990.45	$8.602 \pm 0.036$	$7.696 \pm 0.012$	$6.626 \pm 0.020$			SET=52, This paper
1990.74	$8.697 \pm 0.039$	$7.714 \pm 0.015$	$6.630 \pm 0.020$			SET=57, This paper
1990.82	$8.704 \pm 0.036$	$7.716 \pm 0.012$	$6.634 \pm 0.018$			SET=62, This paper
1990.92	$8.652 \pm 0.037$	$7.719 \pm 0.015$	$6.648 \pm 0.019$			SET=65, This paper
1991.38	$8.695 \pm 0.036$	$7.736 \pm 0.015$	$6.639 \pm 0.019$			SET=67, This paper
1991.46	$8.667 \pm 0.034$	$7.721 \pm 0.011$	$6.625 \pm 0.018$			SET=70, This paper
	$8.659 \pm 0.046$	$7.713 \pm 0.014$	$6.636 \pm 0.011$	$5.816 \pm 0.010$		Weighted mean for SAO 50313
SAO 50205
		$7.330 \pm 0.035$	$7.350 \pm 0.022$			Ljunggren & Oja 1964
	$7.070 \pm 0.031$	$7.290 \pm 0.026$	$7.330 \pm 0.022$			Deutschman et al. 1976
			$7.320 \pm 0.010$			Straizys et al. 1989
		$7.307 \pm 0.012$	$7.340 \pm 0.010$			Turon et al. 1992
1987.76	$7.004 \pm 0.039$	$7.313 \pm 0.035$	$7.330 \pm 0.031$	$7.259 \pm 0.026$	$7.343 \pm 0.032$	SET=28, Paper  II
1989.36			$7.334 \pm 0.016$	$7.272 \pm 0.017$		SET=38, Paper  II
1989.91	$7.065 \pm 0.006$	$7.283 \pm 0.012$	$7.351 \pm 0.009$	$7.287 \pm 0.011$		SET=47, This paper
1990.40	$7.082 \pm 0.034$	$7.339 \pm 0.009$	$7.348 \pm 0.021$			SET=50, This paper
1990.46	$7.080 \pm 0.037$	$7.340 \pm 0.015$	$7.343 \pm 0.019$			SET=53, This paper
1990.75	$7.088 \pm 0.033$	$7.344 \pm 0.013$	$7.348 \pm 0.019$			SET=58, This paper
1990.81	$7.083 \pm 0.032$	$7.339 \pm 0.012$	$7.348 \pm 0.019$			SET=61, This paper
1990.92	$7.086 \pm 0.035$	$7.341 \pm 0.015$	$7.345 \pm 0.021$			SET=64, This paper
1991.41	$7.090 \pm 0.034$	$7.349 \pm 0.013$	$7.349 \pm 0.020$			SET=69, This paper
1991.48	$7.083 \pm 0.032$	$7.349 \pm 0.012$	$7.349 \pm 0.019$			SET=71, This paper
1991.77		$7.345 \pm 0.015$	$7.351 \pm 0.020$			SET=76, This paper
1991.84		$7.349 \pm 0.012$	$7.352 \pm 0.020$			SET=79, This paper
1992.38		$7.350 \pm 0.013$	$7.352 \pm 0.020$			SET=80, This paper
1992.48		$7.347 \pm 0.013$	$7.354 \pm 0.020$			SET=82, This paper
1992.78	$7.069 \pm 0.033$	$7.346 \pm 0.015$	$7.350 \pm 0.020$			SET=88, This paper
1992.89	$7.071 \pm 0.034$	$7.346 \pm 0.018$	$7.351 \pm 0.018$			SET=90, This paper
1993.45	$7.070 \pm 0.032$	$7.347 \pm 0.013$	$7.351 \pm 0.018$			SET=92, This paper
1993.56	$7.068 \pm 0.033$	$7.345 \pm 0.013$	$7.353 \pm 0.019$			SET=95, This paper
1993.72	$7.064 \pm 0.034$	$7.341 \pm 0.014$	$7.345 \pm 0.019$			SET=100, This paper
1993.81	$7.074 \pm 0.034$	$7.334 \pm 0.017$	$7.353 \pm 0.023$			SET=104, This paper
1993.91	$7.067 \pm 0.032$	$7.326 \pm 0.017$	$7.340 \pm 0.022$			SET=105, This paper
1994.48	$7.072 \pm 0.032$	$7.342 \pm 0.011$	$7.347 \pm 0.019$			SET=106, This paper
1995.49	$7.057 \pm 0.032$	$7.349 \pm 0.012$	$7.353 \pm 0.018$			SET=111, This paper
1995.76	$7.070 \pm 0.033$	$7.338 \pm 0.012$	$7.345 \pm 0.018$			SET=112, This paper
1995.84	$7.072 \pm 0.033$	$7.340 \pm 0.015$	$7.349 \pm 0.020$			SET=113, This paper
1995.90	$7.073 \pm 0.032$	$7.343 \pm 0.015$	$7.356 \pm 0.020$			SET=114, This paper
	$7.068 \pm 0.010$	$7.338 \pm 0.016$	$7.345 \pm 0.010$	$7.280 \pm 0.010$	$7.343 \pm 0.032$	Weighted mean for SAO 50205
SAO 50260
	$9.280 \pm 0.040$	$8.200 \pm 0.040$	$7.070 \pm 0.040$			Landolt 1975
			$7.040 \pm 0.010$			Straizys et al. 1989
	$9.230 \pm 0.019$	$8.190 \pm 0.016$	$7.040 \pm 0.013$			Oja 1991
		$8.200 \pm 0.034$	$7.070 \pm 0.031$			Turon et al. 1992
	$9.239 \pm 0.019$	$8.193 \pm 0.004$	$7.043 \pm 0.009$			Weighted mean for SAO 50260

4. Long-term mean correction

The long-term mean brightness correction for any C₁ (hereafter LTM-correction) consists of two parts. First, the UBVRI magnitudes of C₁ used in deriving the O magnitudes during any previous study are subtracted, and the "original'' $\Delta m_{{O}-C_1}$ obtained. Then, the corrected long-term means of C₁ are added to these $\Delta m_{{O}-C_1}$. Here the corrected long-term means for any C₁ of V 1794 Cyg are the weighted means of Table 2. The reasons for the LTM-correction are evident. Firstly, the improved UBVRI magnitudes of C₁ are used. Secondly, consistent long-term differential photometry relies on a constant C₁ brightness. Were different UBVRI magnitudes of C₁ used during different subsets, the mean brightness level of O would be inconsistent. Thirdly, C₁ is not measured during every SET. Thus the brightness must be assumed being equal to the long-term mean determined during other subsets. Our laborious C₁ and C₂ analysis for V 1794 Cyg could have been avoided, had the same combination been consistently used. In the future, only one thoroughly tested C₁ and C₂ combination should be used in the differential photometry of V 1794 Cyg. When the brightness of this combination is accurately determined, procedures like the LTM-correction become unnecessary.