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Astron. Astrophys. 333, 565-570 (1998)

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2. The data

2.1. LTPV uvby photometry, ESO

The uvby data were obtained at ESO in the framework of the "Long-term Photometry of Variables" (LTPV) project which was initiated more than a decade ago (Sterken 1983, 1994). A total of 311 datapoints (i.e. nightly averages of 1-3 measurements) have been collected. Table 1 gives the most important results for each star, as well as the overall averages in [FORMULA], [FORMULA], [FORMULA] and [FORMULA], together with the corresponding standard deviations of individual measurements. The data in Table 1 are based on data from "System 7" (see Sterken 1993) only, and they give a general impression of the photometric accuracy of the LTPV programme. The standard deviations of the programme star greatly exceed those of the comparison stars (note that a single strongly-deviating result obtained on JD 2447490.51 for R 40 was not taken into account, as were two other measurements on JD 2446642.94 and 2448429.94). Comparison star A can be regarded as constant, but star B may be a microvariable; both are of later spectral type than R 40 and, consequently, have redder colour indices than R 40 (which, however, is at its reddest near maximum brightness).


[TABLE]

Table 1. Program star R 40 (P) and comparison Stars (A, B): average [FORMULA] and standard deviations [FORMULA] (in millimag.). N denotes the total number of observations of each star. Note that the results are based solely on data belonging to System 7 Sterken et al. 1993, see also Sterken 1993)


The photometric data were published by Manfroid et al. (1991, 1994) and Sterken et al. (1993, 1995), and we refer to these papers for more details on the observing strategy and on the reduction procedure. All our figures are based on data in the instrumental photometric system.

2.2. LTPV Strömgren by photometry, ESO

R 40 has also been observed during a time span of 33 days in October-December 1995, and over 64 days in 1996. The data are, unfortunately, rather sparse, and were collected with the ESO 50 cm telescope in the y and b bands only. A total of 57 new measurements have been obtained by C. Sterken, B. Vos, I. Zegelaar, H. Melief, A. Kelz and M. Storm.

2.3. VBLUW photometry, ESO

The VBLUW photometry of R 40 was made between 1987 and 1989 with the 90 cm Dutch telescope equipped with the simultaneous VBLUW Walraven photometer. A general description of the monitoring campaign (including the observing strategy and reduction procedures) of luminous and massive stars that included R 40, is given by van Genderen et al. (1985). A total of 85 nightly averages were obtained with a very small mean error of [FORMULA] in V. The sole comparison star used was HD 10747 ([FORMULA], B3V), a standard star of the Walraven photometric system. The mean systematic difference between the Walraven V and LTPV y magnitudes amounts to 0:m001 with 95% of those data deviating less than 0:m003 from this average. Table 2 gives the Walraven and LTPV equivalent V data, and also the corrected magnitudes [FORMULA] that exclude the contribution from the SD phase (see Sect.  4).


[TABLE]

Table 2. Walraven and LTPV V data for R 40 (in magnitudes). [FORMULA] is V corrected for the S Dor phase (see text), HJD is Heliocentric Julian Date minus 2440000 [electronic table]



[TABLE]

Table 2. (continued)



[TABLE]

Table 2. (continued)


2.4. Hipparcos data

132 photometric measurements have been obtained with the Hipparcos photometric instrument (see van Leeuwen et al. 1997). The Hipparcos data, though based on a passband much wider than the Strömgren y band, can be very well combined with our y data by adding -0:m05 to the [FORMULA] magnitudes (see Fig. 1). There is a good agreement with our LTPV data, but the Hipparcos data seem to show a slightly larger scatter. The combined data set provides an almost continuous photometric coverage.

[FIGURE] Fig. 1. LTPV y data ([FORMULA]) together with Hipparcos [FORMULA] data ([FORMULA]) brought to the same scale

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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