Astron. Astrophys. 322, 679-686 (1997)

2. Observations

The observations reported here were done in the period 1994 September 25.0 to 27.4 U.T. with the 6-channel spectrophotometer (Florentin Nielsen 1983) attached to the Danish 1.54 m telescope at La Silla. The photomultipliers in four of the channels independently and simultaneously measure the magnitudes in the Strömgren uvby system.

The observation procedure was similar to that described in Kristensen & Gammelgaard (1993). The four comparison stars were observed approx. every 16 minutes during the night, and the extinction coefficients could therefore be accurately determined by Bouguer's method. Before and after the uninterrupted lightcurve observations photometric uvby-standard stars were observed and by correction with the above extinctions the uvby mean differences between comparison stars and standard stars could be determined. Table 1 gives uvby values of the photometric standard stars and Table 2 the mean uvby values of the comparison stars which were determined by adding the above mean differences to the values of Table 1 for HD 16031 and HD 22054.

Table 1. Extinction Standard Stars. The Strömgren uvby values have been derived from the V, b-y, m1, c1 values in Gray & Olsen (1991) and Schuster & Nissen (1988) by the transformation:
y = V - 0.026 (b-y)

Table 2. Comparison Stars. Mean Strömgren uvby values observed

When HD213985 was observed it turned out that the standard errors of the mean uvby differences to the comparison stars were about twice the standard errors of the corresponding differences between the other two photometric standards (HD16031, HD 22054) and the comparison stars. A search in the Strasbourg data base showed HD213985 as a star varying in V between 9.2 and 10.0.

The differential observations of the planet are given relative to the average magnitude of the comparison stars with equal weights (Kristensen & Gammelgaard (1993)) as given above. The magnitudes of this mean star is given in Table 2. The airmass is eliminated by star observations before and after the observations of the planet. This method gives high accuracy and no discontinuities as one never shifts to other stars. To eliminate second order, color dependent extinction the comparison stars were chosen to match the color of the planet. We also relied on the small bandwith of the uvby-system. However, in the future we should use weights of stars such that

and minimize with the additional condition .

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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