7. Conclusions

Achieving something comparable to the sunspot-record for any late-type star requires uninterrupted and coordinated photometric observing programs. The CaII H&K emission measurements for about 100 solar-type main-sequence stars already extend over a quarter of a century (e.g. Wilson 1978; Saar & Baliunas 1992), and more than a decade of photometry of these stars has been made (e.g. Radick et al. 1990; Radick 1992). The necessity for long-term photometric programs for an extensive sample of all types of chromospherically active stars (e.g. pre- and post-main-sequence stars) has been emphasized, e.g., by Hall (1991). The automatic photoelectric telescopes could perform such a coordinated effort, but only smaller samples have been studied (e.g. Strassmeier & Hall 1988a,b; Henry et al. 1995). Systematic collection and easy availability of photometry obtained with automated telescopes should therefore receive utmost attention. The data time span could be extended with photographic plate archives (e.g. Hartmann et al. 1979). Very intensive photometry of V 1794 Cyg has been carried out, and our data now covers about 20 years. In observing times, the increment from Paper  I is about 207%. The number of individual UBVRI measurements has increased even more ($\sim$286%).

Absolute photometry relies on a selected set of standard stars, but differential photometry requires constant brightness for C₁ and C₂. The best combination for differential photometry of V 1794 Cyg is: C₁ = SAO 50313 or SAO 50205, and C₂ = 57 Cyg. No short- or long-term photometric variability was detected in these comparison stars. Not only will this combination simplify new photometry of V 1794 Cyg, but more importantly, it ensures the homogeneity of all collected photometry. The accuracy in U for SAO 50313 is lower than for SAO 50205. The colour index difference between an early-type C₁ (i.e. SAO 50205) and a late-type variable does require secondary extinction corrections in differential photometry, especially in UB. Yet, the choice of a late-type C₁ (i.e. SAO 50313) may have unfortunate consequences. For example, Hall (1976) discusses the case, where the long-period variable HK Lac was adopted as C₁ for the RS CVn star AR Lac.

Our collection of V 1794 Cyg photometry, as well as of all C₁ and C₂, was terminated on 1995. Analysing 114 subsets would have otherwise been impossible, because any new C₁ or C₂ measurements would have required LTM-corrections in several subsets, i.e. revision of Table 4. For example, who could have foretold the planetary companion detection in one of our C₂, namely 16 Cyg B (Cochran et al. 1997)? For the purposes of our study, the "good news'' about 16 Cyg B are that we used this C₂ only once, and that planetary transit detections in stellar photometry are difficult (Henry et al. 1997). Finally, it was impossible to determine the accuracy of every individual UBVRI measurement of V 1794 Cyg in Table 4. Hence we conclude that reasonable mean external accuracy estimates for all collected photometry are $0\hbox{$.\!\!^{\rm m}$ }015$ in BVRI and $0\hbox{$.\!\!^{\rm m}$ }030$ in U. A prolonged time series of homogeneous standard Johnson UBVRI photometry of V 1794 Cyg has been collected and pre-processed. These data are analysed in Paper  III.

Acknowledgements

The work was partially supported by the EC Human Capital and Mobility (Networks) project "Late type stars: activity, magnetism, turbulence'' No. ERBCHRXCT940483. Dr. M. Seeds and the APT Service are acknowledged of high quality photometry. This research has made use of the Simbad-database operated at CDS, Strasbourg, France.