6 Conclusions

For the six young stars VX Cas, BH Cep, BO Cep, SV Cep, RZ Psc, and WW Vul we have presented photometric data covering a period of about one century. Most of the data consist of brightness estimates on photographic plates from the collections of Harvard and Sonneberg observatories. Additional data come from both published and unpublished multicolour photoelectric observations as well as from visual brightness estimates collected by AAVSO. All the stars are variables of the UX Ori type.

These comprehensive data sets were used to make a comparative study of the light variations of the stars. The major results can be summarized as follows:

1.

The light variations besides the Algol-like minima are different among the stars and vary from a pronounced wave-like pattern (SV Cep) to a nearly constant normal light (BO Cep);

2.

With the exception of BO Cep, no period could be found in the light variations;

3.

The period found for BO Cep confirms an earlier finding by Wenzel (1991). The orbital period of the binary is synchronized with the occurrence of Algol-like minima;

4.

Algol-like minima were observed with all stars. Their number varies among the stars and during the time intervals covered by the observations;

5.

The amplitudes and durations of the Algol-like minima agree with the concept of a cloudy circumstellar shell;

6.

Multicolour observations of individual minima of VX Cas, SV Cep, WW Vul, and RZ Psc prove dust extinction as cause of the Algol-like minima. The reddening law points to larger grain radii than in interstellar space;

7.

Modelling the infrared excess by spherically symmetric dust shells shows that the density distribution follows a power law with a exponent of -1.0 to -1.6;

8.

Apart from BO Cep, infrared observations call for circumstellar shells that are generally compatible with the properties derived from the light variations.

Acknowledgements

The authors are indebted to Dr. I. Shapiro, director of Harvard College Observatory, for the permission to use the plate collection. Three of us (C.F., J.G., H.-G.R.) thank Dr. M. Hazen, curator of the plate collection, for her kind support and hospitality during their stays. We wish also to thank Dr. C. la Dous, director of Sonneberg Observatory, for the permission to use the plate collection. In this research, we have used, and acknowledge with thanks, data from the AAVSO International Database, based on observations submitted to the AASVO by variable star observers world-wide. Furthermore, we would like to thank Dr. V.S. Shevchenko for providing us with his extensive UBVR data set of VX Cas. Finally, we thank our referee, J. Lub, for his constructive remarks that helped to improve the presentation of this paper. The work was partially supported by Deutsche Forschungsgemeinschaft (Fr 963/3-1) and the German Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (Förderkennzeichen 053JN13A).