2 The star sample

In Table 1 we summarize relevant data for the stars studied here. Originally, these stars were taken from the Third Catalog of Emission-line Stars of the Orion Population by Herbig & Bell (1988) by requiring that they should show photoelectrically observed Algol-like minima. Accepting the suggestion by Wenzel (1969) that these Algol-like minima originate from occultations of the stars by circumstellar dust clouds, we have also required that the stars should show an excess of near-infrared radiation as evidence of thermal emission from circumstellar dust. An additional constraint dictated by practical considerations was that the apparent magnitude of the stars had to be brighter than the plate limits during their minima.
Although RZ Psc is neither contained in Herbig & Bell's catalogue nor an IRAS point source, it was included in our study because it frequently shows deep Algol-like minima of the UX Ori type (see, e.g., Zaytseva 1985). It deserves particular interest because of its late spectral type. The non-detection by IRAS does not argue against the presence of an circumstellar dust shell because the emission of a dust shell similar to those around WW Vul and SV Cep is well below the detection limit of IRAS (Friedemann et al. 1994a).

Figure 1: Identification maps for the comparison stars for VX Cas, BH Cep, BO Cep, and SV Cep adopted from the GSC (2000). North is on the top and east to the left. The distance between tic marks on both axes corresponds to $5\hbox {$^\prime $ }$ on the sky. The J2000.0 coordinates of the centres of the fields are $\alpha = 00^{\rm h}\,31^{\rm m}\,20^{\rm s}$ and $\delta = 61\hbox {$^\circ $ }\,59\hbox {$^\prime $ }\,00\hbox {$^{\prime \prime }$ }$ for VX Cas (upper left); $\alpha = 22^{\rm h}\,01^{\rm m}\,00^{\rm s}$ and $\delta = 69\hbox {$^\circ $ }\,40\hbox {$^\prime $ }\,00\hbox {$^{\prime \prime }$ }$ for BH Cep (upper right); $\alpha = 22^{\rm h}\,16^{\rm m}\,54^{\rm s}$ and $\delta = 70\hbox {$^\circ $ }\,03\hbox {$^\prime $ }\,49\hbox {$^{\prime \prime }$ }$ for BO Cep (lower left); $\alpha = 22^{\rm h}\,22^{\rm m}\,00^{\rm s}$ and $\delta = 73\hbox {$^\circ $ }\,36\hbox {$^\prime $ }\,40\hbox {$^{\prime \prime }$ }$ for SV Cep (lower right)

   

Table 1: Fundamental data of the sample stars

Star	$\alpha_{1950}$	$\delta_{1950}$	IRAS	Spectral type	Type of	Amplitude
	($^{\rm h}$ $^{\rm m}$ $^{\rm s}$)	($^\circ$ $^\prime$ $^{\prime\prime}$)			variability	(mag)
VX Cas	00 28 40.4	61 41 17.3	00286+6142	A0/3e	Isa	10.5-13.3 (V)
RZ Psc	01 06 56.0	27 41 30	-	K0IV	Isb:	11.3-13.8 (V)
WW Vul	19 23 50	+21 06 30	19238+2106	A0V	Isa	10.25-12.94 (V)
BH Cep	22 00 38.2	69 30 06.3	22006+6930	A/F5Ve$\alpha$	Isb	10.8-12.7 (V)
BO Cep	22 15 40.2	69 48 44.4	22156+6948	F2e$\alpha$	Insb	12.0-13.7 (B)
SV Cep	22 20 34.0	73 25 16.9	22205+7325	A0e$\alpha$	Isa	10.35-12.15 (V)

References: Kholopov (1985); Thé et al. (1994).