Astron. Astrophys. 337, 393-402 (1998)

2. Observations and reductions

The four objects were observed with the 90-cm Dutch telescope equipped with the simultaneous VBLUW photometer, at the ESO in Chile. Further particulars on the observing procedure can be found in Paper I. Observations in the LMC were made for R 85 from 1988 to 1991, for R 110 from 1989 to 1991, both with respect to the comparison star HD 33486 (B9 , 7:m 9).

Both objects were also observed in the uvby system by the LTPV (Long-Term Photometry of Variables) group (Sterken 1983). R 85 was observed between 1983 and 1994 (136 data points) with respect to the comparison stars HD 35293 (A1 , 9:m 2) and HD 35294 (G2 IV , 8:m 4). R 110 was observed between 1989 and 1994 (72 data points) with respect to the comparison stars HD 37722 (A1 V , 8:m 9) and HD 35294 (G2 IV , 8:m 4).

The observations of R 42 were made between 1987 and 1990 and of R 45 between 1986 and 1989 with respect to the common comparison star HD 10747 (B3 V , 8:m 2), which is also a standard star of the VBLUW system. Table 1 lists the four programme stars as observed in the VBLUW system, the aperture used and average standard deviation () per datapoint relative to the comparison star, all in intensity scale. Average mean errors are of course smaller, in these cases by about a factor two or three. The 's for and of R 110 are relatively large because of its late spectral type due to the (temporary) evolution to the red (this long-term trend has been discussed by van Genderen et al. 1997b).

Table 1. Aperture used (Ap, in arcseconds) and the average standard deviation () per data point (in units of 0.001 intensity) for the four programme stars

Table 2 lists the photometric results in the VBLUW system for the comparison stars and the programme stars. The photometric parameters V and of the UBV system (with subscript J) were transformed with the aid of formulae given by Pel (1987), see van Genderen et al. (1992).

Table 2. The average photometric parameters of the common comparison stars and the four programme stars (in intensity scale for the VBLUW system and in magnitudes for the transformed UBV parameters [with subscript J]). N is the number of measurements.
Notes:
¹ Feast et al. (1960)
² Close to the maximum of an S Dor phase, the colours are still growing red. The photometric parameters are averages from October 1989 until February 1991 (JD 244 7777-8301)
³ Zickgraf (quoted by Wolf 1992)
⁴ Photometric parameters from Pel (1989)

The uvby data of R 85 and R 110 can be found in the four data catalogues by Manfroid et al. (1991, 1994) and Sterken et al. (1993, 1995). The VBLUW differential intensities and colours relative to the comparison stars will be published in a data paper which also includes Walraven photometry of other, previously studied  Cygni variables (to be submitted to the Journal of Astronomical Data).

The variable R 85 was simultaneously (i.e. within a few days) observed in both photometric systems, so that a comparison could be made of the magnitudes. It appeared that in three time intervals (JD 244 7462-7496, JD 244 7772-7943 and JD 244 8119-8290) the average differences (VBLUW) minus (uvby) showed a slight trend from -0:m 031 to -0:m 020 to -0:m 016. Whether the trend is significant is difficult to say, but the effect could be the result of systematic different centering techniques in a star-crowded field by different observers. Anyway, the values derived from the VBLUW system and used in the light curves were corrected for these differences to get a better match with results in the uvby system.

All figures depicting the VBLUW light- and colour curves are given in intensity scale.

© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998
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