3. Comparison of observations obtained in
different sites and using different diaphragms

Even though different sites, instruments, photometers and diaphragms were used in these observations, these data match very well with each other, especially for the colours y and b, for which no discrepancy is noticed between the observations of Tables 2, 3 and 4. The v magnitude differences obtained at PDO (instrumental system) lay systematically fainter than those obtained at ESO by , and the PDO u measurements are systematically fainter than the ESO ones by (again in the instrumental system). The PDO observations were obtained with a single-channel photometer and each filter was measured at a time, while the SAT observations were obtained simultaneously measuring the four colours each time. Despite this difference, the reduction procedures are essentially equal and could not account for the these discrepancies.

One reason for such differences could be the fact that the photometers are essentially different: not only the sets of filters are different, but the PDO photometer is filter defined, while the SAT one has, besides the filters, a grating and a slit in the light path, defining the bandwidth of the measurements (a spectrograph-photometer).

In fact, the u filter transmission curve for the PDO instrument is broader in the "red'' end than that for the filter used in SAT photometer. In Fig. 2 (click here) we show the transmission functions for the filters of both the PDO and SAT photometers. The most striking difference happens for the u filter. Considering that C₁ (Table 1 (click here)) is cooler than V 3903 Sgr, there will be an excess of u light, proceeding from the variable in the PDO measurements in comparison with the SAT ones. This could explain the artificial third light (1.4%) found for the PDO u light curve in Vaz et al. (1997, no third light was found necessary for all the SAT light curves and the PDO vby ones). On the other hand, it is difficult to use the same argument to explain the fact that the PDO u light curve be systematically fainter then the SAT one. This happens in the instrumental system only; the light curves transformed to the standard system do agree in both sets for all four colours, showing that the transformations of Table 5 (click here) do eliminate the problem. However, in order to keep errors in the geometrical parameters as small as possible, we avoid the transformation to the standard system and analyse the light curves only in the instrumental system.

   

Table 5: Transformation coefficients; indicates the differences in the instrumental system (Tables 2, 3 and 4) and the transformed values in the standard system

  
Figure 2: Transmission curves for the uvby filters used at PDO (dashed line) and at ESO (continuous line). The slit limits of the SAT photometer are also shown as vertical dotted lines

A red leak in the PDO u filter could also be the reason for these problems, but we do not have any information about the existence of such a leak. Another possible explanation for such discrepancies might be the different altitudes of the 2 sites: while PDO is at 1 800 m in a very humid region, La Silla is located at 2 400 m above sea level in a much drier climate. This would have stronger effect for the shorter wavelengths.

V 3903 Sgr illuminates the bright nebulae IC 4685 (Hirshfeld & Sinnot 1982, 1985), and it is surprising the negligible effect of using different diaphragms in ESO observations (Tables 3 and 4) on the light curves. As the region is rich of bright nebulae, measurements (especially with SAT, which has automatic pointing, but also manually with the PDO telescope) of the sky background were taken careful and consistently in the same relative positions and after every observation of both variable and the four comparison stars. This indicates that the background contributions, at the places selected for its measurements, are fairly constant in counts per unit area, the only explanation for the excellent agreement between the data obtained with SAT using the different diaphragms.

Coefficients for transformation to the standard uvby system are given in Table 5 (click here) for the data obtained in the two sites and in the different runs. The differences discussed above are reflected in the coefficients of Table 5 (click here).

Further discussion of these observations, including times of minima, some observations in H, and a spectroscopic study will be published as part of a detailed photometric analysis of V 3903 Sgr, based on these uvby light curves and index measurements (Vaz et al. 1997).

Acknowledgements

We acknowledge grants from the Danish Natural Science Research Council, the Danish Board for Astronomical Research, the American National Science

Foundation (AST94-1715), the Wisconsin Alumni Research Foundation, the Wisconsin Space Grant Consortium, the ESO traveling fund, and from the Brazilian institutions CNPq, FAPEMIG, FINEP, CAPES. Support was also received through the programme of cultural, educational, and scientific cooperation between Spain and Denmark. LPV gratefully acknowledges the hospitality received at the Dept. of Astronomy, Univ. of Wisconsin, Madison, from Aug. 93 to Oct. 94. This research has made use of the Simbad database, operated at CDS, Strasbourg, France.