2 The data

  
2.1 Pre-1982 data

The data for the years 1902-1907 are from von Prittwitz (1907). The observations were done with a Zöllner-type photometer (see Zöllner 1861; Sterken & Staubermann 2000); the comparison star was BD+36 3955 = 29 Cyg, a $\lambda$ Boo star in the $\delta$ Scuti instability strip with a pulsation amplitude of 0 $.\!\!^{\rm m}$03 in V and $P \sim 45$ min. The resulting m_v values were not corrected to the V scale. The m_v light curve in Fig. 1 shows four blocks of data, totalling 38 measurements. The mean light level is m_v=5.04 The associated standard deviation (0 $.\!\!^{\rm m}$11) is rather high, and reflects the uncertainties inherent in visual photometry. There is a rather strong increase in brightness level between the first group and the following ones; the overall brightness gradient is about 0 $.\!\!^{\rm m}$02y^-1 over almost 2000 d. The only structure visible is a maximum around JD 2415898, and another one around JD 2416700 (Fig. 1).

A second set of early data was published by Nikonov (1937, 1938): 65 data points, B filter close to Johnson B, the standard deviation (0 $.\!\!^{\rm m}$035) is low, and there is a steady increase of the brightness level by $\sim$0 $.\!\!^{\rm m}$03 y^-1 over the 800 days spanning the observations. Figure 2 gives the B light curve for the recorded measurements. Two clearly-delined light maxima are present, viz. JD 2428074.8 and 2428442.

Figure 1: mv light curve of P Cyg, data from von Prittwitz (1907).

A third set is by Groeneveld (1944), and consists of 55 V measurements collected over a time interval of about five months. Most remarkable is the fast decline in light starting on JD 2431007, and lasting for three subsequent nights with a gradient of $-0\hbox{$.\!\!^{\rm m}$ }023$d^-1. These data are illustrated in Fig. 3.

Figure 2: B light curve of P Cyg, data from Nikonov (1937, 1938).

Percy & Welch (1983) published 11 V measurements of P Cyg (Fig. 4), apparently, a light maximum occurred close to JD 2445123.7 (June 1982).

Figure 3: V light curve of P Cyg, data from Groeneveld (1944).

Figure 4: V light curve of P Cyg, data from Percy & Welch (1983).

 

 

Table 1: Sources of the post-1982 data, ID is the identification label used in the discussion, N denotes the number of measurements. APT stands for Automatic Photometric Telescope.

ID	N	Reference
P	38	Percy et al. (1988)
VRI	95	From VRI APT data
APTA	515	Armagh programme on the APT
APTL	127	Leiden programme on the APT
CAMC	118	Carlsberg Automatic Meridian Circle
DB	17	Dietmar Böhme, Nessa, Germany
Mi	95	R. Milton, Somes Bar, CA
ZS	31	E. Zsoldos, Konkoly Observatory Budapest
PS	8	Peter Sterzinger, Australia
MT	11	Markova & Tomov (markova

Figure 5: Combined V light curve. The continuous line represents the best-fitting sine curve with P=1630 days and amplitude 0 $.\!\!^{\rm m}$015 (fit on the basis of all data collected after JD2446000).

 

 

Table 2: Differences ( APTA minus other) of quasi-simultaneous observations. N is the number of such data avaiable, $\Delta$ is the average difference, $\sigma$ is s.d.

ID	N	$\Delta$ $\phantom{00}$	$\phantom{00}$$\sigma$
VRI	15	-0 $.\!\!^{\rm m}$040	0.051*
APTL	44	-0 $.\!\!^{\rm m}$010	0.011
CAMC	33	0 $.\!\!^{\rm m}$044	0.030
DB	4	-0 $.\!\!^{\rm m}$043	0.015
MTL	5	-0 $.\!\!^{\rm m}$031	0.008
Mi	13	0 $.\!\!^{\rm m}$015	0.021
ZS	8	-0 $.\!\!^{\rm m}$023	0.029

^* After removal of erroneous measurements.

  
2.2 Post-1982 data

Figure 6: V light curve of P Cyg ($\bullet$), times of maximum (Table 4) are indicated by upward arrows, times of minimum (Table 3) by downward arrows. Adjusted Hipparcos magnitudes are represented by +.

Figure 7: V light curve of P Cyg.

The post-1982 dataset is a combination of V data from different sources, as is described in Table 1. The so far unpublished photometric data used in this study will be submitted for publication to the Journal of Astronomical Data (JAD, 2001). Evidently, one may not anticipate that all these data can be swiftly merged into one composite light curve: the various equipment, sites, observers and comparison stars must inevitably lead to inhomogeneities and systematic errors. In order to minimize such effects, we have attempted to determine a transformation from each data set to the largest data subset, which is APTA. We point out that the APTA data were obtained using 32 Cyg as comparison star and 22 Cyg as check star, whereas for the APTL data, 22 Cyg was the comparison star, and 32 Cyg the check star.

We searched all data sets of Table 1 for measurements that were obtained within 0 $.\!\!^{\rm d}$5 from an observation of group APTA. Table 2 gives the result.

It is clear that system VRI yielded the largest instabilities, followed by CAMC. For most other datasets the application of a single zero-point shift ($\Delta$) yields an internally consistent sequence of V magnitudes that are in accord with the data in APTA. Especially for the VRI data, it appeared that the difference is not constant in time.

In order not to degrade any APTA data by quasi-simultaneous data coming from other groups, we have used those other data only whenever no APTA data were available. In this way, we obtained the overall 1982-1999 light and colour curves displayed in Fig. 5, and the major part of the same light curve shown in greater detail in Figs. 6 and 7.

The latter figures illustrate a shape of the light curve of P Cyg that seems quite characteristic: there is a pseudo-cyclic behaviour with a characteristic time of the order of 16-19 days. The descending branch after maximum seems to be quite smooth while the rising branch frequently displays some kind of downward bump just preceding maximum-the latter sometimes has the shape of a stillstand as is often seen on the same location in the light curve of Mira-like variables, although pronounced depresssions (looking like small minima) are seen too.

Figure 5 reveals - besides short-term variability on time scales of 16-19 d and $\sim$100 d - a strong and almost cyclic fluctuation of the mean brightness level, with a characteristic time of about 1500-1600 d or 4 years. Note that the fitted curve (almost 3 cycles) was obtained on the basis of data taken after JD2446000 only. This long-term fluctuation probably can be identified with the S Dor phases, which are typical for these stars and have a more or less cyclical appearance. Note that, considering the residuals around the sine curve, the cycle of 4 y is not quite unambiguous, see Sect. 6 point 3. In addition, an underlying long-term brightness increase with gradient 0 $.\!\!^{\rm m}$007 y^-1 is also present.

  
2.3 Hipparcos data

The Hipparcos catalogue lists 154 reliable $H_{\rm p}$ magnitudes obtained between JD2447859 and 24449046. Only 10 of these measurements have been made within half a day from our data. The broad-band $H_{\rm p}$magnitudes, as is well known, are based on a very broad passband. Hence a correction is needed to bring the $H_{\rm p}$ magnitudes to the same scale as our ground-based data. The average Hipparcos magnitude is $4.8674 \pm 0.0016$, whereas the corresponding ground-based data yield an average $V=4.8043 \pm 0.0014$, hence a correction of the $H_{\rm p}$magnitudes with $-0\hbox{$.\!\!^{\rm m}$ }063$ is in order. Although these Hipparcos magnitudes do not add a single cycle to the data we present here, they are most useful in filling gaps between observing sequences.