The comparison of the new data to the homogenized data from the Hauck & Mermilliod (1998) compilation in the sense (HM minus this paper) is given in Fig. 1. Figures 2 to 5 present the comparison of the data obtained in this paper to the individual sources used in the HM compilation for the considered field, in the sense (others minus this paper). The agreement between H photometry from HM and the present paper is in general good (Fig. 1). The mean differences Delta = (SAT) listed in Table 3 are below 0.010 mag for the majority of the sources and the comparison to the individual sources present small systematic trends of mag only in two cases. For some sources the V magnitudes show a systematic shift in the zero point of as much as 0.03 mag (Fig. 2). This shift is more significant towards the brightest stars (V<7 mag) and does not obviously depend of the colour term (b-y) (Fig. 3). The largest disagreements are with Hill & Perry (1969) and Eggen (1983): 0.032 and 0.019 mag, respectively. The agreement in (b-y) is within the photometric errors, although for some sources a systematic difference of 0.01 mag may exist (Fig. 4). The m1 and c1colour differences obtained in the present paper show a good agreement with Crawford et al. (1971), Schneider & Weiss (1988) and Kaltcheva & Georgiev (1994) (Fig. 5). There is a discrepancy of up to -0.02 in m1 and up to 0.02 mag in c1 with the other sources. Dealing with O and B luminous stars, one should keep in mind the variability in light among them (Abt 1957), which also contributes to the scatter between the different sets. Spectral anomalies can also cause large mean errors in magnitude or colour indices (Young 1974; Manfroid 1985; Franco 1994).
The comparisons in Tables 3 and 4 suggest differences in the precision and accuracy of the different data sets. The consistency of different uvby data sets has been already discussed by Manfroid & Sterken (1987), who stressed the need for a more strictly defined standard system and for closely matching instrumental system. The indication of a small systematic difference in m1 and c1 is slightly worrying. A similar difference in c1 in a large data set has been previously noticed by Franco (1994). Recently, Crawford (1999a) also presented a comparison between uvby data from different publications, showing differences in m1 and c1 up to mag. He also pointed out the reasons that can lead to systematic errors (Crawford 1999b). An additional source may be not applying a correction of -0.008 in c1 to a large set of the southern standards (cf. Olsen 1983). In the case of photometry in star-forming regions, the discrepancy may also be due to the difficulties in selecting a suitable set of primary and secondary uvby standards among luminous and reddened OB stars (cf. Paper I).
The systematic differences between the existing data sets lead to systematic differences in the photometry-derived stellar parameters. The ideal case to compare data only internally is often not possible. In this event the possible inconsistence should be carefully estimated. If the photometric diagrams including c1 and m1 are used, the differences in the photometry, mentioned above, can be easily misinterpreted in terms of luminosity or metallicity. In case of calculating the reddening, a systematic differences of 0.01 in b-y and 0.02 in c1 lead to a difference of 0.01 in E(b-y). In calculating the absolute magnitude MV, which is a function of and c0, the indices are the most critical parameter. A systematic difference of 0.03 mag in , leads to a systematic differences in the corresponding MV values of 0.3 mag. A systematic difference from the standard system of +0.02 0.02 mag in c1 alone leads to overestimation of MV of 0.15 mag and to overestimation of the distance of about 5%.
The structure of the Carina Spiral Feature field based on photometry presented here will be discussed in a forthcoming paper.
This research has been supported by the Danish Natural Science Research Council. This research has made use of the Simbad database, operated at CDS, Strasbourg, France.
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