2. Observations and reductions
The photometry of the four programme stars was obtained with the 90-cm Dutch telescope equipped with the simultaneous Walraven VBLUW photometer, at ESO in Chile, between 1988 and 1991. Observations were made for R 99 from 1989 to 1991, for R 103 and R 123 from 1988 to 1991 and for R 128 from 1987 to 1990, with for the latter also a few observations in 1983, 1986 and 1987.
A description and the aim of the monitoring programme, the observational technique and the achieved accuracy - with mean errors (m.e. = ) in the order of for the average of the nightly means relative to the comparison star - can be found in van Genderen et al. (1985). The effective wavelengths and the band widths are given by de Ruyter & Lub (1986). The Balmer limit is located in the L band, the U band lies at the short wavelength side of the Balmer jump, the W band lies in the Balmer continuum. The photometric data in the VBLUW system are given in intensity scale.
V and can be transformed in the equivalent values of the UBV system ( and ) by formulae given by Pel (1987); see also van Genderen et al. (1992).
Table 1 lists the programme stars, the aperture used and average standard deviation () per datapoint relative to the common comparison star HD 33486 (B9, ), all in log intensity scale. Average mean errors are of course smaller, in these cases by about a factor two to three.
Table 1. Aperture used (Ap, in arcseconds) and the mean error (in units of 0.001 log intensity) for the comparison star and the four programme stars.
Table 2 lists the photometric results for the comparison star and the four programme stars. The differential intensities and colours relative to the comparison star are listed in Table 3, to be published, together with data for other LBVs, in a forthcoming article in The Journal of Astronomical Data.
Table 2. The average photometric parameters of the common comparison star and of the four programme stars (in log intensity scale for the VBLUW system and in magnitudes for the transformed UBV parameters (with subscript J). N is the number of measurements.
Table 3. (available electronically in a forthcoming article of The Journal of Astronomical data) Differential brightness and colours in the VBLUW system of R 99, R 103, R 123 and R 128 relatively to the common comparison star in log intensity scale.
© European Southern Observatory (ESO) 1998
Online publication: March 30, 1998