1 Introduction

The circumstellar dust shells around young stars have attracted much attention in recent years. A particularly interesting group is the so-called UX Orionis stars. In addition to other components, the light variations of these variables show short-lived minima which remind of the lightcurves of Algol stars. Following a suggestion by Wenzel (1969), these minima are widely attributed to the occultation of the stars by circumstellar dust clouds. While the infrared excesses reveal the presence of circumstellar dust in most cases, the Algol-like light variations are strong evidence for the shells being dominated by an ensemble of individual dust clouds rather than characterized by a more or less smooth distribution of the circumstellar matter.

The light variations due to varying circumstellar extinction open a unique way to study the structure of the circumstellar dust shells, the properties of the individual dust clouds, and the characteristics of the dust grains (see, e.g., Friedemann et al. 1994a, 1995; Reimann et al. 1997). For instance, the discussion of multicolour photoelectric data of individual Algol-like minima may provide information on the optical properties of the dust grains in individual circumstellar clouds (e.g., through the reddening law). However, only the study of the long-term behaviour of the light variations provides information on the statistical properties of the cloud ensemble and the structure of the circumstellar envelope. For this aim, we have launched a programme to investigate the long-term behaviour of selected UX Ori stars.

Since there has been no dedicated monitoring of UX Ori stars, the only way to get complete lightcurves over a long interval of time is to make use of photographic plate collections. The richest plate collections are located at the Harvard College Observatory and at Sonneberg Observatory. Most of their stocks is due to the systematic sky patrol work that was done at Harvard from 1898 to 1989 with a break from 1954 to 1962 and that has been done at Sonneberg since 1926 with only a short break in 1995. Thus, the archives of both observatories complement each other and exploiting both of them provides data for a time interval of nearly 100 years.

The aim of the present paper is twofold. First, we will report the results of our brightness estimates on photographic sky patrol plates in the archives of Harvard College Observatory for VX Cas, BH Cep, BO Cep, SV Cep, and RZ Psc and of Sonneberg Observatory for VX Cas, BH Cep, BO Cep, and SV Cep. In the case of RZ Psc the photographic data obtained from the Sonneberg Observatory plates were already published by Home et al. (1994). Second, we wish to present a comparative study of the light variations of these stars with particular emphasis on the Algol-like minima. We will include the UX Ori star WW Vul, for which we have already presented a similar data set elsewhere (Friedemann et al. 1994b, 1996).