1 Introduction

The first list of young early-type stars consisting of 26 objects was published 35 years ago by Herbig (1960). Later new lists of so-called Herbig Ae/Be (HAEBE) stars were suggested by Finkenzeller & Mundt (1984)(55 objects), Shevchenko (1989) (87 objects) and recently by The et al. (1994) (287 HAEBE stars and related objects). During the last 35 years numerous spectroscopic and photometric investigations for individual HAEBE stars, as well as for the representative groups of these stars, have been carried out (see for example The et al. (1994). Unfortunately polarimetric investigations of young early-type stars are very limited. Up till 15 years ago only three attempts to study the polarimetric characteristics of HAEBE stars - based on the original Herbig list - were made (Vrba 1975; Garrison & Anderson 1978; Vrba et al. 1979).

In the following 10 years no polarimetric observations of groups of young early type stars were reported, except for the paper by Petrova & Shevchenko (1987)(14 objects). In 1986 Grinin and collaborators started a program of simultaneous polarimetric and $U\!BV\!RI$ photometry of photometrically active HAEBE stars with Algol-like minima (Grinin et al. 1988; Voshinnikov & Grinin 1988; Berdyugin et al. 1990, 1992 etc.). In the 1990s polarimetric studies of HAEBE stars were reported by Jain et al. (1990), Hutchinson et al. (1994) and more intensively by Jain et al. (1995), but these observations also included only limited lists of HAEBE stars (8, 11 and 24 respectively). In relation to the total number of HAEBE stars and candidate members The et al. (1994), the number of HAEBE stars which have now be investigated polarimetrically is small ($\approx \, 30\%$; Yudin 1988, 1992). Moreover, with few exceptions, no circular polarization measurements have been carried out for a sample of HAEBE stars to date. This deficit of polarimetric data does not allow us to judge the polarimetric characteristics of HAEBE stars as a definite group of young objects.

Polarimetric variability is detected in most investigated HAEBE stars (Vrba 1975; Garrison & Anderson 1978; Vrba et al. 1979). Some of the objects show variability on a long time-scale: years, months (Scarrott et al. 1989; Jain et al. 1990, 1995). Scarrott et al. (1989) found changes in position angle (PA) for R Mon on a time-scale of years and assumed that they were caused by the precession of the circumstellar (CS) disc around the object. Other HAEBE stars with Algol-like minima show variability on a time-scale of days and an increase in the degree of polarization is accompanied by a decrease in visual flux (Grinin 1994). Such variations are explained in terms of nonperiodic eclipse of stellar light by rotating "protoplanetary" condensations. Note that, in all the above cases, the main origin of polarization is scattering by dust grains in CS shells. However, it is well known that HAEBE stars are also surrounded by extended and dense gas shells, which may also contribute to the observed polarization. The short time-scale polarimetric variability which has been established for a few HAEBE stars (Dzhakusheva et al. 1988; Beskrovnaya et al. 1995) can be explained by Thomson scattering. Unfortunately up to now we have had no information on polarimetric variability on time-scales of hours or less, except for the few cases mentioned above. Thus, the investigation of polarimetric variability on different time-scales might give us additional information on the mechanisms of polarization.

Bastien (1985, 1988) showed that there exists a correlation between the polarization and infrared (IR) colour indices for young T Tau stars (TTS). More detailed investigations of such correlations were carried out by Yudin (1988) for a sample of TTS and HAEBE stars. He showed that a strong linear correlation exists between $\log p$ and IR excess for all types of young stars of different spectral classes. One of the conclusions from Yudin's (1988) work was that the above mentioned correlations can be used for selecting stars at the pre-main sequence (PMS) stage of evolution. However, deviation of an object's position from the above correlations for young stars can be caused not only by the different evolutionary status of objects but also by different orientations of disc-like dust structures around the stars.

A further important factor is the relation of HAEBE stars with other groups of peculiar early type stars, such as Be and B[e] stars, luminous blue variables (LBVs) etc. The evolutionary status of a significant group of B[e] and Be stars is controversial; are they PMS stars, or at a later stage of evolution (see e.g. the investigation of the B[e] star MWC349 (Yudin 1995))? In many cases polarimetric properties of HAEBE stars and B[e] (Be) stars are similar.

  

Table 1: Herbig Ae/Be stars and candidate members included in the present study

  

Table 2: B[e] stars, extreme emission line objects and other early-type emission line stars with IR excess included in the present study

Taking into account all of the above factors we have carried out a polarimetric study of peculiar early-type stars with the following aims:

1.

First, to obtain new polarimetric (linear and circular) data for a large group of HAEBE stars and candidate members, as well as for some peculiar Be and B[e] stars and LBVs. The stars observed in the present programme are listed in Tables 1, 2. Table 1 contains HAEBE stars and candidate members, while Table 2 contains B[e] stars, extreme emission-line objects and other early-type stars with IR excesses. In addition two TTS (S CrA and RY Lup) were included in our study. The whole list of programme stars consists of 60 objects located mainly in the southern hemisphere; most were taken from The et al. (1994). For 40 of the 60 objects polarimetric data have been obtained for the first time.

2.

Second, to study polarimetric variability of the programme stars on different time-scales (minutes ...hours ...days). For this reason most of the investigated stars were observed at least twice. Some objects were observed repeatedly during a night and/or monitored during a period of a few hours. Also, to study variability on long time-scales, the data presented here were compared with previously published data.

3.

Third, to compare the polarimetric characteristics for a significant group of HAEBE stars and related objects with their photometric data. It is well known that most HAEBE stars show significant IR excess due to CS dust (Berrilli et al. 1989; Hillenbrandt et al. 1992). However the configuration of CS shells around HAEBE stars is currently a point of discussion. At present it is not possible to conclude whether these shells have spherical symmetry or, perhaps, have a disc-like structure. In the case of optically thin discs any IR excess does not depend on the viewing aspect. On the other hand, if the main origin of polarization is scattering in a dust disc-like structure, the degree of polarization will strongly depend on the inclination of the discs (see e.g. McLean & Brown 1978). For this reason we selected for our programme stars with different IR excesses in order to search for a link between polarization and IR excess. The range of observed flux in the V and L bands from the literature is given in Tables 1, 2.

In Sect. 3 data obtained for individual objects are briefly discussed. In some cases we present an estimate of brightness for individual objects based on our data. A statistical study of the polarimetric properties of the selected group of stars is presented in Sect. 4. In Sect. 5 we discuss the relationship between the polarimetric and photometric characteristics of some objects and draw conclusions about the orientation of CS discs around them.